File Extensions

File extensions are usually three or more letter codes or numbers at the end of computer file names that tell the operating system (Microsoft Windows 7, Windows Vista, Windows XP, Windows 95/98/ME/2000, MAC OS X, Linux/Unix operating systems etc.) what kind of data file they are dealing with.
When you use programs to write a letter, edit a photo or build a spreadsheet, the program must assign an extension so the computer knows what program it should use to open it. This is one of the big differences between Windows and Mac. A Mac can only use Apple programs so the computer knows which program to use, but Windows has many programs created by independent suppliers so it requires the extension to keep track. The 3 or 4 letters are for the humans and the computer has an instruction in the first line of code for the computer to understand.

Common Extensions: There are over 1800 extensions in use and more coming everyday, but here a few common types.
Programs Extensions that open programs or start tasks:
*.exe, *.bat, & *.com are program starting files. It is an executable file - that is, it tells the computer to start the program.
WARNING! Don't launch executable files that you receive from an unknown source (such as via an unsolicited e-mail), since the sender could have a malicious purpose. Also, many worms propagate via e-mail address books, so you may know the purported sender of an infected file. Don't open any executable file until you have confirmed by phone or e-mail that the file is legit.

Letter Extensions used to save documents:
*.doc, *.rtf, *.txt, *.docx, & *.wpd are files of documents like letters and text.

Photos Extensions:
*.jpg, *.tif. *.bmp, *.png & *.pic are files for photos and pictures.

Linking Extensions that attach a file to a program:
*.dll, are data linking libraries that contain functions for another software program.

Web Extensions are web pages for your browser:
*HTML, *.XML and *.SHTML contain the information need to display a web page.

To solve the problems with all the extensions, websites have been set up to keep track of the extensions in use. One file-extension organization is at http://www.file-extensions.org/. Programmers are asked to list any new extensions they create on this site. Another website is www.filext.com.
In addition many programs can open and save files with different extensions. They convert the file format to the new type. When you use the Save As button when saving a document, a little down pointing triangle to the right of the "Save As Type" box will show the possible variations you can use.
For example, Works word processor can save your document as:
*. wps – works version 2003,
*.wps - Works version 2000,
*.wps - Works version 4,
*.rtf - formated text ,
*.doc – Word 97 -2000,
*.csv - comma separated values,
*.HTML – web pages.

This allows you to save in a format that your reader can open if he doesn't have the same version as you and it also allows you to open earlier versions of Works documents.
Computer viruses sometimes masquerade as harmless e-mail attachments. The fastest way to spot the interlopers is by their file extension. Knowing a file's extension can also help you customize your system and clean out unused files and also help open email attachments.

Unfortunately, Microsoft started hiding file extensions in Windows 95. To make sure your file extensions are visible, open Windows Explorer or any folder window and choose View, Folder Options or Tools, Folder Options (depending on your version). Click the View tab, then make sure the option to 'Hide extensions for known file types' is unchecked (the exact wording will vary depending on your version of Windows; see FIGURE 1), and click Apply, then click OK. Now extensions will be visible for nearly every file on your desktop and in your Explorer and folder windows.
Being unable to open a certain file on a personal computer is a very typical situation for lots of users. You click on a file – or an email attachment - and a pop up comes up:
"Windows cannot open this file: "
To open this file, Windows needs to know what program you want to use to open it. Windows can go online to look it up automatically, or you can manually select from a list of programs that are installed on your computer.
In order to open the file, you will need the correct software installed on your PC. For example, if the file you are trying to open requires Microsoft Excel, you will either need to purchase Microsoft Excel, or download the free Microsoft Excel file viewing program from Microsoft's web site. Many software manufacturers offer free file-viewers for their software that allows you to view the file. If you want to edit or change the contents of the file, you will have to purchase the full software package.
Figure out what application was used to create your file.
Look at the file, then go online to www.filext.com and type the extension in their search box. This site will tell you what type of program will open this file.
It's possible that you already have the software that you need installed on your PC. If you have the correct software, but you still see the "Windows cannot open this file" error, you may have to re-associate the file type to that program.
How to create a file association in Windows XP
From the error message that appears, click "Select the program from a list" and click "OK".
In the window that appears, select the correct program to open the file, and click the "Always use the selected program to open this kind of file" box.
Click OK
Windows XP will now always use the program you selected to open this kind of file.
If all this fails, contact the sender of the email and tell them you can't open their attachment and ask what type of program they used to create the file.