Switch to `save_msg_file()` + `MessageObject::file_name` for saving attachments

[iequidoo]

Currently save_msg_file() isn't used for saving attachments (iiuc the app implements copying of MessageObject::file on its own). Also i didn't understand why, but when i use "Save as", i see that not the original filename is used in the dialog (i.e. MessageObject::file_name) by default, but the last component of the blob path (Message::file), so when i send two files with the same name and then try to save the second file, it suggests the mangled filename. Core's Message::set_file() should always set Param::Filename to the original filename, so i have no idea why that happens.

[Simon Laux]

MessageObject::file_name does not output the correct original filename for some reason, maybe it is a bug with the composer, because it recreates the message everytime you modify the draft and on sending.