I was trying to move a lot of verbose text to an attachment to keep the main short but Posterous simply appending the text files to the end of the post. I am not sure there is an easy way to detect text attachments consistently across all e-mail clients and a MIME-aware e-mail library that is flexible enough to handle it.

These are notes for 1 e-mail client, Gmail.

The e-mail was sent MIME encoded. The first layer was split into 3 parts with multipart/mixed. The actual blog post was wrapped inside a second layer as multipart/alternative.

I. Content-Type: multipart/mixed
   A. Content-Type: multipart/alternative
      1. Content-Type: text/plain; charset=ISO-8859-1
      2. Content-Type: text/html; charset=ISO-8859-1
   B. Content-Type: text/plain; charset=US-ASCII
   C. Content-Type: text/plain

The actual attachments have Content-Disposition: attachment and Content-Transfer-Encoding: base64, so it is reasonably easy to see when viewing the raw text.

Probably the most important question is even if text attachments were supported, how would they be displayed? Will an HTML iframe work elegantly like the supported file types listed in the FAQ at http://posterous.com/faq/?