When Mail merely says an attachment is a MIME attachment, it means that there is inadequate file format information to say more. Most, if not all, attachments you will receive in the normal course via email will be MIME, but when the extension or other file format information is included, it will not mention MIME.
If the file was created on a PC, and does not have an extension (such as .jpg, .doc, .pdf, etc.), then Mail nor any other email client will know what the file type is supposed to be.
What are the files you are receiving supposed to be?
Ernie