Thumbnails for ingredients can't be located in local path starting with /c2pa/ because signing fails with bad parameters error message

[ktuszek]

When a thumbnail for an ingredient is located in a local path starting from /c2pa/... signing fails with:

Error: embedding manifest

Caused by:
    bad parameter: hash is missing

an example of ingredients specification that fails:

  "ingredients": [
    {
      "title": "Title",
      "relationship": "inputTo",
      "thumbnail": {
        "format": "image/png",
        "identifier": "/c2pa/thumbnail/subtitle_thumbnail.png"
      }
    }
  ]

[mauricefisher64]

External thumbnails are not supported

[mauricefisher64]

Reopening because I misread your intent.