During MIME analysis, MIME readers classify all non-multipart MIME entities and "multipart/appledouble" MIME entities (that contain appropriate child MIME subparts) into the following three categories:
MIME entities that represent non-inline attachments.
MIME entities that represent attachments that can potentially be inline.
All MIME entities that can be classified as attachments (category 2 or 3) SHOULD be treated by MIME readers as attachment MIME parts, and an entry in an attachments table SHOULD be created for each such MIME part. However, depending on the value of the Content-Type header, analysis SHOULD be done differently, as follows:
"message/rfc822" MIME entities SHOULD be treated as embedded message attachments, as specified in section 18.104.22.168.3.
"Multipart/appledouble", "application/applefile", and "application/mac-binhex40" MIME entities SHOULD be treated as Macintosh attachments, as specified in section 22.214.171.124.2.
"Message/external-body" attachments SHOULD be treated as external body attachments, as specified in section 126.96.36.199.
All other attachments SHOULD be treated as regular file attachments, as specified in section 188.8.131.52.1.
If no Content-Type header is present on a MIME entity, MIME readers SHOULD treat it as a "text/plain" MIME content-type (unless this MIME entity is a subpart of multipart/digest, in which case the default value for the Content-Type header is "message/rfc822").