Add API extension and setting for providing filename to post attachments

To restore some lost functionality, I propose an API extension to solve the issue of filenames being lost and not displayed, nor user configurable. Some people use filenames when sharing media as part of the joke/punchline and cannot do this anymore.

1. Add a new field to post attachments similar to the description field. Apps/FEs will be able to populate it as they wish.

  "media_attachments": [
    {
      "description": "pumpedupkicks.mp4",
      "id": "879900874",
      "pleroma": {
        "mime_type": "video/mp4",
        "filename": "new filename metadata goes here.png"  ⬅️⬅️⬅️⬅️
      },
      "preview_url": "https://lain.com/media/cb75de0200dd5c7d2dff673cacb632649694daf83aff269363e135121fc4d26e.mp4",
      "remote_url": "https://lain.com/media/cb75de0200dd5c7d2dff673cacb632649694daf83aff269363e135121fc4d26e.mp4",
      "text_url": "https://lain.com/media/cb75de0200dd5c7d2dff673cacb632649694daf83aff269363e135121fc4d26e.mp4",
      "type": "video",
      "url": "https://lain.com/media/cb75de0200dd5c7d2dff673cacb632649694daf83aff269363e135121fc4d26e.mp4"
    }

2. Federate and ingest this data on posts

3. Add new user account setting to enable this functionality which will allow an app or FE to display the custom field when authoring posts with attachments and render the attachment filename links (if metadata available) in the timeline.

cc @hj for additional comments or requirements.