Fix bridge->bridge file names

WhatsApp uses the "title" and "fileName" fields for document names. The bridge
was only reading title and sending fileName, so sending a document from one
bridge to another through WhatsApp would lose the file name.

Also use names like "image.png" instead of "<random id>.png" for unnamed files

go.mod

@@ -16,4 +16,4 @@ require (
 	maunium.net/go/mautrix v0.7.13
 )
 
-replace github.com/Rhymen/go-whatsapp => github.com/tulir/go-whatsapp v0.3.11
+replace github.com/Rhymen/go-whatsapp => github.com/tulir/go-whatsapp v0.3.12

go.sum

@@ -111,6 +111,8 @@ github.com/tulir/go-whatsapp v0.3.10 h1:LfzRj6V3dshKemad3HZsgYgKAZtaT0wNITmobJYA
 github.com/tulir/go-whatsapp v0.3.10/go.mod h1:U5+sm33vrv3wz62YyRM/VS7q2ObXkxU4Xqj/3KOmN9o=
 github.com/tulir/go-whatsapp v0.3.11 h1:z9AvTb8YXYP8t6Y0jium3reshgDpFCg//WNiCGo9esQ=
 github.com/tulir/go-whatsapp v0.3.11/go.mod h1:U5+sm33vrv3wz62YyRM/VS7q2ObXkxU4Xqj/3KOmN9o=
+github.com/tulir/go-whatsapp v0.3.12 h1:4ovBn1GNMUsCrFR7KdAcByQnkh0o+VoA7Q/wY2UD36A=
+github.com/tulir/go-whatsapp v0.3.12/go.mod h1:U5+sm33vrv3wz62YyRM/VS7q2ObXkxU4Xqj/3KOmN9o=
 golang.org/x/crypto v0.0.0-20180904163835-0709b304e793/go.mod h1:6SG95UA2DQfeDnfUPMdvaQW0Q7yPrPDi9nlGo2tz2b4=
 golang.org/x/crypto v0.0.0-20190308221718-c2843e01d9a2/go.mod h1:djNgcEr1/C05ACkg1iLfiJU5Ep61QUkGW8qpdssI0+w=
 golang.org/x/crypto v0.0.0-20200604202706-70a84ac30bf9 h1:vEg9joUBmeBcK9iSJftGNf3coIG4HqZElCPehJsfAYM=

portal.go

@@ -241,10 +241,14 @@ func (portal *Portal) handleMessage(msg PortalMessage) {
 			length: data.Length,
 		})
 	case whatsapp.DocumentMessage:
+		fileName := data.FileName
+		if len(fileName) == 0 {
+			fileName = data.Title
+		}
 		portal.HandleMediaMessage(msg.source, mediaMessage{
 			base:      base{data.Download, data.Info, data.ContextInfo, data.Type},
 			thumbnail: data.Thumbnail,
-			fileName:  data.Title,
+			fileName:  fileName,
 		})
 	case whatsapp.ContactMessage:
 		portal.HandleContactMessage(msg.source, data)
@@ -1471,7 +1475,13 @@ func (portal *Portal) HandleMediaMessage(source *User, msg mediaMessage) {
 	}
 
 	if msg.fileName == "" {
-		msg.fileName = msg.info.Id
+		mimeClass := strings.Split(msg.mimeType, "/")[0]
+		switch mimeClass {
+		case "application":
+			msg.fileName = "file"
+		default:
+			msg.fileName = mimeClass
+		}
 
 		exts, _ := mime.ExtensionsByType(msg.mimeType)
 		if exts != nil && len(exts) > 0 {
@@ -1895,6 +1905,7 @@ func (portal *Portal) convertMatrixMessage(sender *User, evt *event.Event) (*waP
 		info.Message.DocumentMessage = &waProto.DocumentMessage{
 			ContextInfo:   ctxInfo,
 			Url:           &media.URL,
+			Title:         &content.Body,
 			FileName:      &content.Body,
 			MediaKey:      media.MediaKey,
 			Mimetype:      &content.GetInfo().MimeType,