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It most often consists of 3 or 4 characters. Thanks to the file extension, the computer that supports a particular file is able to recognize the format in which the data is stored. This in turn results in that the machine can display the data in a manner that can be "understood" by a man.
In addition to the list of applications that support specific file extensions, you can also find on File-Extension.org suggested locations from where you can safely download the necessary software. Most often they are the websites of software manufacturers.
If your goal is to convert the file to another format, you need to find an application that can perform such an operation. Find in our database a file format you want to convert and see what the possibilities are.
Popular file formats have detailed descriptions to identify the specific file extension. All extensions have information about theirs manufacturer, file category, as well as popularity.