Add support for attachment file names

Send, receive, and save attachment filenames. Use them as suggested
names when saving files.

// FREEBIE

js/libtextsecure.js

@@ -39049,6 +39049,9 @@ MessageSender.prototype = {
                 proto.id = id;
                 proto.contentType = attachment.contentType;
                 proto.digest = result.digest;
+                if (attachment.fileName) {
+                    proto.fileName = attachment.fileName;
+                }
                 return proto;
             });
         }.bind(this));