Inline images are very similar to attachments: We just want to store
them somewhere and refer to them. But we want to use a different element
(<img> instead of <a>) or more generally, a different template. So we
pass the disposition as an additional argument to save_part and use it
to construct the template name - which gives as a flurry of new
templates.
Strange combination, but the first message with multipart/alternative
also contained a .scp file and not as an attachment.
The template for multipart/alternative allows switching between the
alternatives.
For now we just save them unmodified and give them an extension that
will cause the web server to provide the correct content-type. This is
probably *not safe*: A user could send malicious html as an attachment
and the browser will interpret it when another user clicks on the link.
We might try to sanitize attachments (but you would normally expect an
attachment to be preserved) or to preserve the content-dispostion header
(but I don't think this is possible with just a static archive).
This doesn't handle the case where total isn't present on all parts, but
I don't actually expect to encounter that (and if I do, it will crash
and I can fix it).