MM-28984: Add fallback text to push notification (#16089)

We read the fallback text from message attachments and append that as part
of a push notification.

https://mattermost.atlassian.net/browse/MM-28984

```release-note
NONE
```

app/notification_push.go

@@ -164,7 +164,7 @@ func (a *App) getPushNotificationMessage(contentsConfig, postMessage string, exp
 	senderName, channelName, channelType, replyToThreadType string, userLocale i18n.TranslateFunc) string {
 
 	// If the post only has images then push an appropriate message
-	if len(postMessage) == 0 && hasFiles {
+	if postMessage == "" && hasFiles {
 		if channelType == model.CHANNEL_DIRECT {
 			return strings.Trim(userLocale("api.post.send_notifications_and_forget.push_image_only"), " ")
 		}
@@ -582,6 +582,10 @@ func (a *App) buildFullPushNotificationMessage(contentsConfig string, post *mode
 		msg.FromWebhook = fw
 	}
 
+	for _, attachment := range post.Attachments() {
+		post.Message += "\n" + attachment.Fallback
+	}
+
 	userLocale := utils.GetUserTranslations(user.Locale)
 	hasFiles := post.FileIds != nil && len(post.FileIds) > 0