Is there a way to filter posts based on the content of the MIME header?

I've just discovered that my list allows something through which triggers a
flood of rejections because of security concerns. It was an attachment of
the .url type.

The problem is that we need to allow attachments with the MIME Content-Type
of "application/octet-stream"; we exchange files that are often conveyed
that way in the course of our work. However, if such a file carries a .url
extension, it is (wisely) considered unsafe by many email servers, producing
a flood of rejections. (I've gotten 15 so far, some for multiple
subscribers--and that's only the ones that slipped past the error summary!)

I was hoping, therefore, to filter on the file extension, and just refuse to
allow the string ".url" to be sent at all. However, I can't seem to find a
way to catch it in the content filters. LISTSERV appears to regard material
in MIME headers as neither a header entry (so I can't write a filter
starting with "Content-Type:") or message text (so I can't just filter on
".url" itself).

Is there a way to do this? or is that part of a message just plain immune to
examination by LISTSERV?

Hal Keen