File Association: How Windows Opens Files
File association is what Windows uses to know what program to use to open a specific file. The last part of the file’s name, the filename extension, is used by Windows to identify the file’s type. The file type tells Windows how to open the file, which program to use to display the file’s contents, whether the file is a program, or whether the file doesn’t belong to anything.
For example, the file named My plans to kayak the storm drain.doc has the filename extension .doc, which is short for document. On your computer, .doc files are associated with a program, such as Microsoft Word. The association means that the file’s icon looks like a Word document icon and that opening the file opens the file inside Microsoft Word. That’s how file association works.
Windows keeps track of dozens of filename extensions, noting which programs open those files.
The filename extension is the final part of a filename. It begins with a period and is followed by one or more characters. Most extensions are three or four characters long.
Files without extensions cannot be associated with any program. Likewise, not every extension is known to Windows.
A file may appear to not have an extension, but it’s there.
File association can be overridden manually. So you can choose, on the fly, another program to open a file. Or, you can change the file association permanently.
One major problem with filename association is that installing a new program may re-associate existing files on your computer. For example, programs opened by Paint may suddenly be opened by Photoshop Elements. To prevent this problem, most programs ask before they make the re-association. If not, you can reassign files to associate with any program.
File association based on filename extensions isn’t a good system. It’s relatively easy to change a filename’s extension, which changes the association without changing the file’s contents. This can cause trouble, which is why you’re warned in Windows whenever you attempt to rename or change a file’s extension.