Not recognizing RTF bodies for some formats

[gwiedeman]

Describe the bug For EML and MBOX files, mailbagit currently treats a message part as a body if it does not have a content_disposition stating its an attachment and has mime types text/html and text/plain. It looks like it might be possible for message bodies to have text/rtf or application/rtf mime types, though email clients may or may not support this. Its worth investigating this and how libpff and extract_msg handle RTF.

Additionally, our current content_disposition check may be insufficient, as it does not exclude content_disposition=inline. It would be weird to have an inline text or html part, but its still a good idea to exclude it as a body.