In general, Rebex Mail tries to accept broken input data (such as messages with invalid "image/*" media types), but reject invalid data when constructing new messages.
This might occasionally cause problems in scenarios where new messages are constructed based on incoming messages. However, producing messages with broken content might instead cause problems to receivers.
In this case, changing the media type to a generic one (such as "application/octet-stream") seems to be the best workaround.
Of course, it would be better to fix the software that created messages with "image/*" headers.