A handler can now return an ExternalAttachmentResult ({externalAttachment:
true, html, text}) instead of a file id. The composer then deletes the
staged file, removes the attachment and appends the replacement content
to the body, so a plugin can push a large file to an external store and
leave a download link behind. Returning a string (or nothing) keeps the
stock upload path byte-for-byte.
Also adds plainTextMode to the upload callback deps, which the new branch
reads to pick between the html and text replacement.