There exist files of different types, such as .gif, .txt etcetera. A program, module, function etc may know how to deal with certain file types and not others. Try renaming a file on your computer to have a different extension. The file will probably no longer open normally in the program you used to use. You haven't changed the contents of the file, but you have changed the tag that programs use when they look at files to tell if they're dealing with a file that they can work with.

The Mime type is the type of the file. You can use the file extension to determine the file type, but this is not the most robust mechanism.

MimeType