Enigmail shows signatures from OpenPGP/MIME as normal attachments. While this is technically correct, it does not reflect the normal use case. This makes it impossible to quickly find mails with "real" attachments. You have to open each mail and see if there is another attachment beside signature.asc.
Therefore, Enigmail should not show the signature as normal attachment. Alternatively it should provide a possibility to access the raw signature over the details drop down menu from the info line above a mail.