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No, folders usually do not have file extensions, but they could have if you like. Usually only files have extensions, to indicate the contents of the file.
JPG,GIF,Bmp
They're used as extensions to identify picture files.
MP3 sound files have an .mp3 file extension, and common file extensions for raw, uncompressed sound files are .wav and .aif. Pg. 474
video files (mpg file extension, MPEG file format). It is the same video file format. You can use booth file extensions.
There is list of programs that support DO file extension http://www.howopen.org/en/extensions/web-files/do
File extensions tell us what kind of file it is.For example, a .pdf extension tells us that it is an Adobe Acrobat file.Files with the extensions of .jpg, .jpeg, .png, and .bmt are all picture files. (This is not an extensive list of picture files, just an example).Other types of files like .doc, .dot, .dotx, .docx, are Word files. There are many more file extensions for that.Powerpoint files have extensions like .ppt and .pptx.Windows media files have extensions like .wmv.Every software program has its own extensions that identify what it is.In Windows Explorer (or Apple equivalent), you can organize your files by file type if you're looking for a specific type of file. It's also really helpful if you're tracking statistics on file types and usage, or finding a file that you need to use again.The sort by file type feature is very handy when you have a high volume of files in a directory and you're looking for a document of a specific type.If you don't know where you placed your file, you can do a global search on the file type (if you know it) and your computer will give you search results showing only the files with that extension.
Emails are not files, so they do not have extensions.
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INC file extensions usually indicate that they are "include" files (i.e. they are written with the intent to being called from another source code file). These files are opened using any normal text editor, such as Notepad, Vi, or Edit. Special editors also exist for the language that the source code within is written in, and typically includes auto-completion, syntax highlighting, and error checking.
Rules for file extensions are based mainly on the underlying operating system and/or file system. Generally speaking, file extensions are just a part of the file name, and can be set to whatever you like.