You can use application/octet-stream for unknown types. RFC 2046 states in section 4.5.1: The “octet-stream” subtype is used to indicate that a body contains arbitrary binary data.
I’ve had success with exactly the same headers as you’re using, with the following differences: Embedded is not a valid value for the Content-Disposition header. attachment should be fine. inline should also be fine, though I’ve usually seen attachment for multipart/related embedded images. The value of the Content-ID header is supposed to be in the … Read more