File Extensions Help and Tips

1. What Are File Extensions?

SUMMARY: File extensions defined. This tip explains what the usually three characters are for.

In general terms, a file format in the PC world signifies what type of information is containing in a particular file. These are usually designated by the one to four (usually three) characters (the extensions) after the period in a filename. For example:

Filename: readme.txt
The "txt" designates the file format of this particular file, a plain-text file.

Filename: game.exe
The "exe" designates the file format of this particular file, an executable file.

Note that file extensions do not always signify the type of information inside a file; sometimes programmers hide the type of content inside a file with fake extensions. Also, some filenames may have multiple file extensions that can trick you into opening a Trojan Horse.

2.Viewing File Extensions on Windows

SUMMARY: Help protect yourself from accidentally opening rogue files by forcing Explorer to show all files' extensions.

Depending on how your Windows computer is setup, file extensions may be hidden. Instead of seeing files containing a period and 1-4 character extensions, all you see are the filenames without the extensions. Or, some files may have extensions, but not all of them.

For safety reasons, and to learn more about file formats, you should configure Windows to show all file extensions.

Although the instructions for showing file formats differ depending on the version of Windows you are running, they are generally the following:

1. Open any folder on your machine.
2. Choose "Tools" - "Folder Options".
3. Click the "View" tab.
4. Uncheck "Hide extensions for known types".
5. Click "OK" to close the dialog box.

3. Be Careful with Files Containing Multiple Extensions

SUMMARY: Files with multiple extensions may be Trojan Horses waiting for you to open them and release their nefarious content. Be careful with these files.

Especially on the Internet and when looking at electronic mail attachments, such as inside spam emails, you may come across files with multiple extensions, such as:

readme.txt.exe

In this case, note that the final extension is the actual extension of the file. The "txt" is NOT the file's extension; it is "exe". This means that although you might think the file is a text file, based on the "txt" extension, the file is actually an executable, based on the "exe" extension.

Thus, if you were to double-click this masquerading file, instead of getting a text document opened with Notepad, you would actually be running a problem. And, unfortunately, in this case, the program might be a dangerous Trojan horse (computer virus) that could do damage to your system or others'.

Be VERY WARY of any file containing more than one extension! Although not true 100% of the time, much of the time files with multiple extensions were written to fool people into executing Trojan horses.

4. EXE File Format


A file ending in ".EXE" usually means the file is an executable, or program file. This contains code that the computer follows in order to perform a particular task. A word processor, such as Microsoft Word, is an executable file; when started (double-clicked), the computer reads the file and executes the contained instructions to start up a word processing application. The same can be said for game executables, spreadsheet executables such as Microsoft Excel, music player executables such as Winamp, and so on.

Executables can tell the computer to run virtually anything possible. This means that while most executables are useful for you to perform tasks, play games, access the Internet, etc., computer viruses may also be started by executables. When run, they could format your computer, randomly delete files, send spam e-mail to others, or whatever.

While a program's filename (the part of the file before the period) SHOULD give an explanation of what running the executable will do (for example, WINWORD.EXE starts Microsoft Word and EXCEL.EXE starts Microsoft Excel), filenames can be named virtually anything.

Thus, you should only start executable files that you trust. DO NOT just run any .EXE executable sent to you via electronic mail; it may contain a computer virus that can cause damange to your machine and possibly others'! Even if you are sent a file called WINWORD.EXE, that doesn't mean running it will actually start a copy of Microsoft Word.

5. TXT File Format

A file ending in ".TXT" usually designates that the file is a plain-text file. This can contain virtually any type of text - a recipe, documentation for software, a book report draft, or whatever.

When a file ending in ".TXT" is double-clicked, a text editor, such as Notepad, should appear, allowing you to view and/or edit the contents of the file.

Windows users: NOTE that unless you have configured your computer with the file extension viewing tweak, it is possible that a filename that looks like it ends with ".txt", especially one attached to an e-mail message, actually has a hidden OTHER extension, meaning that if you double-click the file, it may actually run a computer virus!

6. ZIP File Format

The .ZIP extension signifies the file is a ZIP archive. This is a way that people can transmit a game, screensaver, application, electronic book, or whatever as one larger file instead of multiple smaller files. ZIP files are used often in DOS/Windows environments.

ZIP files must be created with a ZIP file creation tool. These tools take a group of files and mash them ("zip" them) together into one "archive" file, sometimes reducing the amount of space the ZIP file takes on the hard drive ("compressing"). After the ZIP file is transferred, the recipient takes the ZIP file and "unzips" it, causing the original files to appear on their hard drive.

Another advantage to ZIP files is that, in certain cases, they can be password protected. This way (theoretically) only the recipient, knowing the password, can unzip the file. Different zip/unzip software packages use different compression mechanisms that may be easier/harder to break.

The following companies provide programs that can zip and unzip files. Some are freeware, others shareware:

* Info-Zip, a freeware ZIP/UNZIP software package that works on multiple platforms.

* PKWARE, the creator of the ZIP file format.

* WinZip, arguably the most popular zip/unzip software for Windows.

Note that ZIP files can contain virtually any type of file, including other ZIP files (you can place a ZIP archive inside of another archive). While most ZIP files contain legitimate text files, executables, photos, music, etc., it is possible for malicious people to distribute rogue ZIP files containing spyware, viruses, and more. Thus, never unzip and use a ZIP file from an unknown recipient!

7. DIZ and NFO File Formats

The DIZ and NFO file extensions, most used in files named filename.diz and readme.nfo, represent text files most commonly used in the bulletin board days that preceded the Internet. These files were used to describe the contents of ZIP archives. Though not commonly used today, you can still sometimes see them when downloading files from certain websites.

8. RAR File Format

RAR is another file format most commonly used on DOS/Windows machines. RAR files take multiple files and pack them into one file, making it easier for people to distribute games, applications, screensavers, photo libraries, etc. Later, the RAR file recipient unpacks the file into its original contents. In this way it RAR files are much like ZIP files.

* WinRAR is a popular software package used for making RAR files.

* Several packages can unpack (but not pack/create) RAR files, including PowerArchiver.

Like RAR files, ZIP files can contain virtually any type of file - good or bad. Never unpack and use a RAR file from an unknown recipient!

9. TTF File Format

The TTF file format, created by Apple, represents a TrueType font.

* What is a font?

Briefly, a font is a representation of all of the characters of the alphabet, plus numbers, special symbols, international characters, or more. When you use a package such as Microsoft Word and change how your letters look from Arial to Helvetica, Times New Roman, Lucida Sans, Impact, or whatever, you are using different fonts.

* What makes TrueType fonts different?

TrueType fonts were created for a couple of reasons, including:

* Fonts can be resized onscreen. A long time ago, if you needed small characters, medium-sized characters, and large characters, you would require three separate files that could render characters in these various sizes. However, with TrueType fonts, one file contains instructions so that characters can be rendered small, large, or any size in between.

* TrueType fonts look the same on your computer screen and your printer. When you create and print a document using TrueType fonts, the characters on your printout will look the same as they do onscreen. A long time ago, this was not the case.

TrueType fonts come with Macintosh and Windows.

10. PDF File Format

The PDF, or Portable Document Format, is a file format created by Adobe Systems. This format represents a way to transmit brochures, booklets, tax forms, restaurant menus, software documentation, or virtually any other type of word processing or desktop publishing document. This file format is popular on most operating systems, allowing people to transmit all types of content without having to use a myriad of different file viewers.

PDF files are usually created with Adobe Acrobat, though other freeware and shareware tools abound. They are normally viewed with Adobe Reader, a freeware application Adobe provides to help continue the use and adoption of PDF files.

11. SCR File Format

The SCR file extension represents a Windows screensaver, the animations that may appear on your computer whenever it has not been used for a few minutes. Screensaver animations can be virtually anything from starfields to flying kitchen appliances to photos of your favorite travel destinations.

Screensavers must be located in a particular folder to be viewable. Most of the time they come with installer software that performs this task for you; some free screensavers, however, must be manually moved to the current folder. For example, in Windows XP, screensavers must be placed in the C:\WINDOWS\system32 folder (or equivalent).

12. TMP File Format

TMP files are usually temporary files that applications such as word processors, games, disk utilities, etc. create and eventually remove. Most of the time these files just contain indecipherable gibberish only usable by the application that created them. However, sometimes the curious can find readable text inside certain temporary files if opened with Notepad or another text editor.

If you see TMP files on your hard drive, they are usually safe to remove if they are older than a few weeks and you have no software currently running on your system. You may wish to back up TMP files first, wait a few days, and then delete them.

13. WAV File Format

WAV (WAVE) files are audio files playable via multimedia playback software such as Windows Media Player and other software available for your operating system. These files may contain spoken words, sound effects, music, or whatever.

WAV files normally take up a lot of room on hard drives and are being used less nowadays, especially for full songs. Other formats can store music taking up a reduced amount of space.

14. AU File Format

Files with the AU extension contain audio data. This file format was created by Sun Microsystems.

In this way, WAV files are similar to the WAV files, but are found most often on Sun, Unix, and Linux-based systems. Since they are uncompressed audio files they take up much space on a hard drive; thus, you will most likely only see them used for small sound effects rather than entire songs.

15. PSD File Format

The PSD file format, created by Adobe, contains graphics/photos created by image editing software such as Adobe Photoshop. This file format is popular as it can be read by Macintosh and Windows computers. Normally PSD files are shared by desktop publishers and image editors as in-progress graphics, and are not used as much for graphics distributed to end-users.

16.BAK or BK File Format

The BAK (or BK) file extension normally represents files that are backup copies of other file. There is no 'standard' file format for BAK files. BAK files can represent virtually anything.

If you're running out of hard drive space, BAK files can sometimes be safely removed if they are a couple of days old and you have closed down all running software. However, before you do so, you should consider what file the BAK file is a backup of; it may be beneficial to keep the file. Examine the filename as well as the date and time the file was created.

If you have background processes running that make copies of certain files by creating BAK files, removing these files negates any usefulness of such processes.

17. BMP File Format


The BMP file extension normally represents that the file is an image. BMP files are normally used on Windows machines though many other operating systems have programs to view such files. BMPs can represent virtually any type of image imaginable, from photographs of cars to holiday clip-art.

BMP files are made up of pixels, tiny dots that represent color in an image. Some BMP files may only contain black-and-white images (1-bit), others images that have up to 16 colors (4-bit), other 256 colors (8-bit), some 65,536 colors (16-bit), and others 16 million (24-bit). The more colors that can be represented in a bitmap the larger the file.

You can shrink a bitmap's size to some extent and still retain much of the image quality. However, if you increase a bitmap's size, no new detail is automatically added. Thus, the more you resample a bitmap upward, the blockier and more jagged it will look as the pixels turn into larger blocks of the same color. This results in 'pixilated' low-quality images.

Virtually any image viewer/editor on Windows machines, and many on other operating systems, can view and edit bitmaps. BMP files are "uncompressed", meaning that computers perform no operations on these files to reduce their file size. Also, it means you can open and resave a bitmap as many times as you want and the file will remain looking the same (assuming you do not adjust the color depth), which is not the case for some other image file formats.

18. LOG File Fomat

The LOG file extension normally means that the file is a text file containing log information created by a particular program. There is no specific format to LOG files; they may contain the dates and times a particular program ran, what tasks it performed, how long those tasks took, etc. And, LOG files are not always text files but may be in a proprietary format depending on the software program that created them.

If you're running out of hard drive space, LOG files can sometimes be safely removed if they are a couple of days old and you have closed down all running software. However, before you do so, you should consider what process created the LOG file, in case you really need to keep the file. Examine the filename as well as the date and time the file was created. For safety, DO NOT double-click LOG files; open them from within Notepad or another text editor to ensure the file contains readable log information and is not some other proprietary file format.

19. MID File Format

The MID file extension normally represents that the file is a MIDI music file. MID files can only contain music, no voices. MID files are different than files such as WAV files in that they do not contain sampled, or digitized, audio information. Instead, MIDI songs only contain several tracks, each of which designates instruments to use and notes to play in a song, as well as miscellaneous musical effects. Thus, MIDI files will sound differently depending on which sound card is used to play a file.

For example, one track of a MIDI song may represent the drums. Another, a bass guitar. Another track may be a grand piano accompaniment, yet another might be a lead trumpet.

MIDI songs are normally created by specialized music software packages such as the following.

20. PPT File Format

The PPT file extension normally represents a Microsoft PowerPoint presentation, or slide show. These are most often created with Microsoft's PowerPoint software, though some other software, such as StarOffice, can also read and modify these files.


* For Windows: PowerPoint Viewer 2003 - this should read PowerPoint files created by Microsoft PowerPoint 97 and later.

* For Macintosh: PowerPoint 98 Viewer.

A word of warning. PowerPoint presentations can include embedded macros. These normally add legitimate functionality to a PowerPoint presentation. However, they sometimes can be malicious. Make sure you only open PowerPoint presentations from trusted sources. If you open/edit presentations using Microsoft PowerPoint, there is a way to adjust macro security settings.