* Add filename param to attach_inline_image
* Add attach_inline_image_file function
(parallels EmailMessage.attach and attach_file)
* Use `Content-Disposition: inline` to decide
whether an attachment should be handled inline
(whether or not it's an image, and whether or not
it has a Content-ID)
* Stop conflating filename and Content-ID, for
ESPs that allow both. (Solves problem where
Google Inbox was displaying inline images
as attachments when sent through SendGrid.)
