MIME type handling in uploading a serving files through attachment_fu

Question

I'm currently using attachment_fu to handle file uploads, and while the plugin provides a nice catchall for images in :content_type => [:image], nothing similar seems to exist for broader buckets. None of the uploads will be executed, per se, only saved to disk and served back at a later time. So a few related questions.

1. Is there any harm in accepting all MIME types? Putting :content_type => nil seems to do it.
2. Does Rails/attachment_fu/some other plugin provide a formidable listing and grouping of MIME types?
3. How can I implement specific callbacks for certain MIME types?
4. I stumbled on mimetype-fu to parse the application/octet-stream uploads. Is this the best practice?
5. Is there, perhaps, a way in Rails to delegate MIME type determination when delivering the files to subsequent users to the server?

Many cheers for any help/guidance.