Filename extension
From Just Solve the File Format Problem
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A filename extension (or file extension) is a suffix of a filename, usually the part after the last "." (period). It is a crude but convenient way to help indicate the format or type of the file.
The details of how, and whether, filename extensions are used differ by operating system, but in the internet age they have developed some degree of universality.
Some operating systems limited extensions to three letters, and many three-letter strings are still in common use as extensions, but the current-day popular systems don't have this limitation and there are many longer extensions in use.
