attachment handling has been simplified, and now use a "register" method similar as the
ones used for download or upload.
A default method (for unencrypted messages) will try a simple upload and will copy the
links to body.
AESGCM plugin has been adapted to be used for encrypted files. If more than one file is
sent with AESGCM plugin, they will be split in several messages as current de-facto
standard (OMEMO media sharing) doesn't support several files per message.
"path" refers to a local path of a file, we don't want to store it in database as the file
may be in a temporary location, or even be a temporary file. So once the file has been
uploaded, we remove it.
