Allow message specific actions for message post processing to privileged users

My requirement:

  • a guest/external user uploads one or multiple files as a message attachment
  • an internal/authorized user should review the file content
  • and select a message or single attachment for post-processing in an integrated app

So I was looking for the possibility to extend the message actions (“Reply”, “Copy Link”, …) by a custom command. The list of actions in mattermost-mobile/post_options.js at master · mattermost/mattermost-mobile · GitHub seems to be fixed, no extension is allowed in there.

Is there another approach to manually trigger post processing of uploaded files for privileged users ?