Don't installing conflicting mime type for *.*64

There is already an mime type for Nintendo64 roms called application/x-n64-rom.
This one has to be extended instead of installing a conflicting type. Otherwise
only one of the desktop entries for mupen64plus-ui-console or m64py works and
not both.
Merge pull request #7 from fayvel/mime

Don't installing conflicting mime type for *.*64