As I think I recently said, this is what UnknownPlug is for. Here's
how I use it to import Quicktime movies, despite there not being a
QuickTime Movie plugin. From collect.cfg:

plugin ImagePlug
plugin UnknownPlug -process_exp '.MOV' \
-assoc_field 'Movie' -file_type 'misc/misc'

Note that the associated file's name will be stored in a metadata
element called movie (and that I don't know the correct mime type!).
My format strings for displaying pages and lists contain expressions
like this (all on one line, of course):

format CL1VList '<td>[link][Title][/link]</td>
<td>{If}{[Image],Image!<br />[Image]}
{If}{[Movie],Quicktime movie!<br />[Movie]}</td>'

Gordon