The Shared MIME Database stores information about file types. It holds information such as "Files with names ending in .html have the MIME type text/html" and "A text/html file should be described as an `HTML Page'".
Normally, you shouldn't need to edit this database, but if you do you can use MIME-Editor for the task.
The documentation is in the MIME-Editor/Help directory.
There is no need to compile; it's a python program.