A custom filetype isn't being recognised, how can I make sure it is?

You can force this through using a '.htaccess' file. Remember that the directives inside an '.htaccess' file apply not only to the directory in which the '.htaccess' file is placed, but also to its subdirectories too.

To override the default file associations and reorient the mapping of given file extensions to user-specified handlers, the AddHandler directive can be invoked

Suppose it is desired that all files with the extension of '.cat' be recognized and processed by the server as a cgi program -

AddHandler cgi-script .cat

If you think a given MIME type is worthy of server-wide recognition, please let us know.