Improve wording of the “Add media” button tooltip (#13954)

- Remove (incomplete) list of supported formats
- List types of media (image, video, audio)
- Tell that several images could be uploaded using plural

app/javascript/mastodon/locales/defaultMessages.json

@@ -1206,7 +1206,7 @@
   {
     "descriptors": [
       {
-        "defaultMessage": "Add media ({formats})",
+        "defaultMessage": "Add images, a video or an audio file",
         "id": "upload_button.label"
       }
     ],