Ever get an email attachment or see a file and not know what program to use for it? Most of the time your computer will figure it out so when you double-click on it, the necessary program will open it up. The extension on the file (what comes after the "." dot), gives a hint as to what the file type is so you know what program you can use to open the file (or if it's a system file you shouldn't mess with) but these days many operating systems will hide "known" file extensions. In this case, "known" means your operating system can identify the file type and should be able to open it. However, you may still want to see the file extensions so you know what the files are too.
To set your Mac (using OSX) to always show file extensions,
Click on Finder, Preferences, Advanced tab
Check the Show all filename extensions box.
To set your PC (using Windows 7) to always show file extensions,
Start, Control Panel, Appearance and Personalization, then Folder Options (Category view) or Control Panel then Folder Options (Icon view)
Click on the View tab.
Uncheck the box that says Hide extensions for known file types.
Once you know what the file type is, you can do a search on the Internet to determine what software is necessary or best to open the file and even if you don't have the necessary program to open it right away, you can keep the file until you do or request the file in a different format (i.e. it came in Word but you don't have Word so ask for it in pdf format so you can view it with the free Adobe Reader program).
The following table lists some of the more common file extensions and what they are. After the table are links to web sites that list more file extensions in a searchable format. There did not appear to be one site that listed them all.
Access 2007 or newer database file
Active Server Page; for web pages
Audio Video Interleave; movie file
Bitmap graphics; picture file commonly created with Paint on Windows
Comma Separated Values file; set up like a text file where columns are separated by a comma; can be viewed with Word or Excel
Dynamic Link Library; a system file often shared by many programs
Document text file created with Word (pre 2007 format)
Word 2007 or newer Macro-enabled Document
Word 2007 or newer Document
Document Template; created with Word, used as a basis for new documents (pre 2007 format)
Word 2007 or newer Macro-enabled Document Template
Word 2007 or newer Template
Encapsulated PostScript vector graphics; picture file
Executable file; for programs that are self-starting like Excel, Access, Word
Graphics Interchange Format; picture file viewable on multiple platforms
.htm or .html
Hypertext Markup Language; for viewing web pages
.jpeg, .jpe, .jpg
Joint Photography Experts Group; picture file
Database file, contains record-locking information for when you have a MS Access database open.
Microsoft database file; used by Microsoft Access (pre 2007 format)
Apple QuickTime Movie
Portable Document Format; file format most computers can read with the Adobe Reader program
Hypertext PreProcessor; for web pages
PowerPoint 2007 or newer Macro-enabled Presentation Template
PowerPoint 2007 or newer Template
PowerPoint Slide show (pre 2007 format)
PowerPoint 2007 or newer Macro-enabled Slide Show
PowerPoint 2007 or newer Slide Show
PowerPoint 2007 or newer Macro-enabled Presentation
PowerPoint 2007 or newer Presentation
Rich Text Format; text file that can be opened with any text editor or word processor
Stuffitt archive; compressed file
Tagged Image File Format; high-end picture file
Text file that can be opened with any text editor or word processor
Waveform audio file; sound file
Word Perfect Document
Excel Spreadsheet (pre 2007 format)
Excel 2007 or newer Macro-enabled Workbook
Excel 2007 or newer Workbook
Excel 2007 or newer Macro-enabled Workbook Template
Excel 2007 or newer Template
For more complete lists or to do a search, check out any or all of the following sites:
Page last updated 01/04/12. Send questions or comments to email@example.com.