I am not sure why it is finding an "asc" file as only a PDF was attached.
The banned files mechanism checks mime parts, not only attachments.
Furthermore, even if I added ".asc" to the list above, it is still flagging the email as having banned content.
A text-file attachment internally transforms to a list of strings:
["multipart/mixed","application/octet-stream",".asc","file.txt"]
Your default policy has to handle those mime-types appropriately. Example of a PDF inside a zip archive:
["multipart/mixed","application/octet-stream",".zip","file.zip",".pdf","file.pdf"]
A plain text body of a mail without attachments:
["text/plain",".asc"]
Your case probably has been:
["multipart/mixed","text/plain",".asc"]
