First, what are extensions?

File extensions are the three or four characters that are after the period in a filename on Windows, Linux and other operating systems. Macintosh does not use file extensions in earlier versions of its operating system but with the introduction of OS X, extensions are becoming more important.

EXAMPLE
Myfilename	.txt
File name	extension

What do extensions do?

File extensions help make a link (or association) between an application and a data file. This file association is the computer’s way of knowing which program will open which file. They are also the way programs like web browsers and email clients know what “helper” applications might be needed for viewing content like Flash movies and MP3 files.

What are some common extensions?

• .doc – short for document, can be opened by Miicrosoft Word and WordPad
• .txt – short for text, can be opened by Notepad, SimpleText and most word processors
• .jpg – short for Joint Photographic Experts Group, can be opened by many graphics programs as well as Netscape Navigator, Internet Explorer and photo editors
• .zip – a compressed file that must be opened with “unzip” software like PKUNZIP, FreeZip or other decompression software
• .exe – short for executable, this is the extension for applications.
• .pdf – short for Portable Document Format, these files are read by Adobe Acrobat Reader
• .dll – short for dynamic link library, these files allow Windows to work smoothly…do not delete them unless you are totally sure.

Some software packages have their own extension to identify their files:

• .xls – Excel spreadsheet
• .psd – Photoshop document
• .wpd – WordPerfect document
• .wps – Microsoft Works document

1. Go to Windows Explorer or open a folder and choose View and then Folder Options.
2. Click the File Types tab. Scroll down the Registered File Types list until you find the file type you want.
3. Click the Edit button and then click the Browse button.
4. Find the folder that has the program you want to use to run this particular file type. (Remember that the program file will end in .exe.)
5. Double click the filename and it appears in the Application Used To Perform Action box. Click OK.

1. Open the folder you want to change
2. Click on Folder Options under the View menu along the top of the window (NOTE: look under Tools in Windows XP)
3. A dialog box will appear. Click on the View tab in the middle. Look down the list of options and remove the check in the box by “Hide file extensions for known file types”. Click OK

Why would I need to know about file extensions?

1. Some file types can be opened by more than one program. Understanding file extensions can help you change what programs open certain files (like sounds or pictures) so your computer will work the way you want it to work.

2. Some viruses are transmitted by files that “spoof” as one type of file but are really another. Look for files with two periods and two extensions (i.e. filename.jpg.pif) Without seeing the file extensions, you might think this file is a picture file, but it could be a file that can launch a virus. Another trick is to use a lot of spaces in the filename so what you see in the filename is not exactly what the file name can be. For example: “filename.jpg .scr” might only show up as “filename.jpg” in your view. Also, some file extensions are more “dangerous” than others. Knowing those can help decide whether to open unexpected attachments from folks you know (or don’t)
FYI: On the new PSTCC mail server, certain extensions are blocked from being sent or received, these include: .exe; .com; .scr; .vbs and others.

3. Knowing file extensions can make it easier to share files with people who may not have the same applications on their computer. For example, using a Rich Text Format (.rtf) file will allow a word processed document to retain its formatting but not require Word or WordPerfect or other specific word processor for access.

File Formats: When you try to open a file unsuccessfully, there are a few options to try:

1. Ask the sender what program they used to create the file. Then, see if you have access to that program or if there is a viewer. Microsoft offers viewers for Word, PowerPoint and Excel documents so you do not have to own those programs to see the files.
2. Ask the sender to use their “Save As” function to make a file in a format that will work for you. Word can save as RTF, WordPerfect and several other formats. Many page layout programs can create Adobe Acrobat files (NOTE: If there is not a Save As function, look for Export as another options)
3. You may have to work with your program to “teach” it to open a certain type of file. This is especially applicable with web browsers, email clients and audio players.

Converters also exist to change files from one format to another:

• For Graphics: Paint Shop Pro, Quicktime Pro and Photoshop all open and export a variety of graphic formats
• For text based documents: Translation CM Plug-In, Dataviz’s Conversion Plus and MacLink Plus Deluxe

If creating a file to send or share, consider who is getting the file. If it is someone who is on campus, then they will most likely have the same type of software as you. If it is someone at another school or location, then you may need to ask what format works best for the documents or files you wish to share.

Easily shared file formats include:

• TEXT: Rich Text Format (.rtf), Adobe Acrobat (.pdf) and Plain Text (.txt)
• IMAGES: JPEG and GIF
• SOUNDS: WAV and MP3
• COMPRESSION: zip (for PC) and .sit (for Mac)

Helpful Resources

What is Every Files Type in the World -This is a long list of a LOT of file extensions along with the program that most likely created them. Handy when you get a file that you cannot open and do not know where to even start.

The Importance of File Extensions – The best thing about this page is his list of “problem” or “dangerous” extensions that can be sent via email. Useful resource.

File Associations and other Tips about Installing and Using Windows – A concise summary of what is said below … plus some more info!