We found a fix for this. Yes it is the MIME issue and the file you need to change is under lib/filelib.php.
The bug is that Moodle is trying to display .ai files as an image but the browser does not recognize it as a image file that it could display. Anyway, all you have to do is to change this line:
'ai' => array ('type'=>'application/postscript', 'icon'=>'image.gif'),
to
'ai' => array ('type'=>'document/unknown', 'icon'=>'ai.gif'),
We also created an ai.gif file which I attached here, for this specific file type.
Maybe the Moodle core team could check this simple hack into 1.5.4+, 1.6.3+, and 1.7+...
Again, as I always said, new features are great, but
usability is king! Make Moodle less buggy would really increase users' confidence and this is very important, especially for open-source software... (my 2 cents)
Hope this helps,