plugin attach, aesgcm: attachments refactoring: 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.