Incorrect file associations could be the result of underlying issues within your PC system. As such it is highly recommended to scan your PC for invalid registry entries, unused processes and other unfavourable system settings to identify slowdown issues.
The MAN file type is primarily associated with 'Manual'.
Often an ASCII text file.
These are often manual pages for UNIX system commands. They are called up on the console and you can scroll through the page to get the necessary information.
Sometimes the page is compressed; thus that MIME type.
Typically lower case on *nix systems.