If you need to perform a default action on a local "download" file, i.e.,if you are using "file://", I would suggest that you manually edit the mimetypes.rdf file and add the particular extension that is causing you such pain.
For web page use and working with MIMETYPES read the below:
From the Firefox/Seamonkey KB on File types and download actions
You may not be able to set an automatic download action. This can occur when a misconfigured web server assigns an incorrect MIME type, such as "application/octet-stream" (shown here in SeaMonkey 1.x)
or if the server assigns "Content-Disposition: attachment" to the file download (as shown here in SeaMonkey 1.x).
When this happens, the option to always perform the same action will either be "grayed-out" or, in Firefox, selecting the "Do this automatically" option will not seem to have an effect and you will again be asked what to do when you next encounter that file type (see below for related bug reports). There is little you can do in these cases because the problem is at the server end.
Note: the MIME type of a downloaded file, and whether it should be handled as an attachment, is only shown in the SeaMonkey 1.x "Opening" dialog. This information does not appear in Firefox or in SeaMonkey 2.
Why are correct MIME types important?
Example of an incorrect MIME type result If the web server or application reports an incorrect MIME type for content, a web browser has no way, according to the HTTP specification, of knowing that the author actually intended the content to be processed and displayed in a way different from that implied by the reported MIME type.
Some other web browsers, such as Microsoft Internet Explorer, try to allow for misconfigured web servers and applications by guessing what the correct MIME type should be. This has sheltered many web administrators from their own errors, since Internet Explorer will continue to process content as expected even though the web server is misconfigured, e.g. displays an image which is reported to be plain text.
Serving content using the correct MIME type can also be important for security reasons; it's possible for malicious content to affect the user's computer by pretending to be a safe type of document when it is in fact not.