discord: support sending files with just a URL

previously, the code assumed each file had a Data, and crashed if this was not present.
now, we detect this condition, and send the URL instead.

the discord client will display URLs in a message of their own
as inline embeds, without any special behavior needed on our part.

this commit makes the same changes to the webhook and non-webhook code.

bridge/discord/discord.go

@@ -380,7 +380,27 @@ func (b *Bdiscord) handleEventBotUser(msg *config.Message, channelID string) (st
 // handleUploadFile handles native upload of files
 func (b *Bdiscord) handleUploadFile(msg *config.Message, channelID string) (string, error) {
 	for _, f := range msg.Extra["file"] {
+		var err error
 		fi := f.(config.FileInfo)
+
+		if fi.URL != "" {
+			err = b.handleUploadFileFromURL(msg, channelID, &fi)
+		} else if fi.Data != nil {
+			err = b.handleUploadFileFromData(msg, channelID, &fi)
+		} else {
+			b.Log.Errorf("Attachment %#v for message %#v had neither Data nor URL", fi, msg)
+		}
+
+		if err != nil {
+			b.Log.Errorf("Could not send attachment %#v for message %#v: %s", fi, msg, err)
+			return "", err
+		}
+	}
+
+	return "", nil
+}
+
+func (b *Bdiscord) handleUploadFileFromData(msg *config.Message, channelID string, fi *config.FileInfo) error {
 	file := discordgo.File{
 		Name:        fi.Name,
 		ContentType: "",
@@ -393,13 +413,18 @@ func (b *Bdiscord) handleUploadFile(msg *config.Message, channelID string) (stri
 	}
 	res, err := b.c.ChannelMessageSendComplex(channelID, &m)
 	if err != nil {
-			return "", fmt.Errorf("file upload failed: %s", err)
+		return fmt.Errorf("file upload failed: %s", err)
 	}
-
 	// link file_upload_nativeID (file ID from the original bridge) to our upload id
 	// so that we can remove this later when it eg needs to be deleted
 	b.cache.Add(cFileUpload+fi.NativeID, res.ID)
+	return nil
 }
 
-	return "", nil
+func (b *Bdiscord) handleUploadFileFromURL(msg *config.Message, channelID string, fi *config.FileInfo) error {
+	_, err := b.c.ChannelMessageSendComplex(channelID, &discordgo.MessageSend{
+		Content:         fi.URL,
+		AllowedMentions: b.getAllowedMentions(),
+	})
+	return err
 }

bridge/discord/webhook.go

@@ -69,6 +69,26 @@ func (b *Bdiscord) webhookSendTextOnly(msg *config.Message, channelID string) (s
 func (b *Bdiscord) webhookSendFilesOnly(msg *config.Message, channelID string) error {
 	for _, f := range msg.Extra["file"] {
 		fi := f.(config.FileInfo) //nolint:forcetypeassert
+		var err error
+
+		if fi.URL != "" {
+			err = b.webhookSendFileFromURL(msg, channelID, &fi)
+		} else if fi.Data != nil {
+			err = b.webhookSendFileFromData(msg, channelID, &fi)
+		} else {
+			b.Log.Errorf("Attachment %#v for message %#v had neither Data nor URL", fi, msg)
+		}
+
+		if err != nil {
+			b.Log.Errorf("Could not send attachment %#v for message %#v: %s", fi, msg, err)
+			return err
+		}
+	}
+
+	return nil
+}
+
+func (b *Bdiscord) webhookSendFileFromData(msg *config.Message, channelID string, fi *config.FileInfo) error {
 	file := discordgo.File{
 		Name:        fi.Name,
 		ContentType: "",
@@ -88,12 +108,22 @@ func (b *Bdiscord) webhookSendFilesOnly(msg *config.Message, channelID string) e
 			AllowedMentions: b.getAllowedMentions(),
 		},
 	)
-		if err != nil {
-			b.Log.Errorf("Could not send file %#v for message %#v: %s", file, msg, err)
 	return err
 }
-	}
+
-	return nil
+func (b *Bdiscord) webhookSendFileFromURL(msg *config.Message, channelID string, fi *config.FileInfo) error {
+	// discord client will display any file url as an inline embed,
+	// without us having to do anything special.
+	_, err := b.transmitter.Send(
+		channelID,
+		&discordgo.WebhookParams{
+			Content:             fi.URL,
+			Username:            msg.Username,
+			AvatarURL:           msg.Avatar,
+			AllowedMentions:     b.getAllowedMentions(),
+		},
+	)
+	return err
 }
 
 // webhookSend send one or more message via webhook, taking care of file