Any fix for wrong mime type encoding for outgoing attachments?

Mail, both in Tiger and now in Leopard, gives some outgoing file attachments (particularly Word and Excel files) the (?default) mime type 'application/octet-stream' instead of the correct mime type e.g. 'application/msword'. This has not always happened (both in Tiger and Leopard) but something has made Mail change its mime type encoding. This occurs even with the correct file extensions, and send windows friendly attachments selected.

Some e-mail clients can still recognize and interpret the attachments correctly, but others do not.

If an attachment is received (or is in a mailbox) that has the correct mime type then it can be dragged to another message and keep the correct mime type. However if it is copied to the desktop and then reattached to a new message it ends up with the octet-stream mime type instead. (If it is dragged from the desktop onto the new message, as opposed to using the attach button, it ends up with a null mime type, but that appears to be a different bug - it did not occur in Tiger.)

Is there a database somewhere that Mail uses to look-up the mime type for a particular file type/extension? If so I might be able to edit it to correct the mime type allocation. A search for file names containing 'mime' using File Buddy shows a number of files, none of which is an obvious candidate. Also some of these files appear to be encoded as opposed to plaintext which would make them very difficult to edit.

Any help would be much appreciated.