Improvement: Also Accept attachment with type Image as Banner image.

Currently when Mobilizon tries to fetch an event from another server the Banner image must be an attachment of type Document:

This image is not recognized:

...
"attachment": [
    {
      "type": "Image",
      "url": "http://wp.lan/wp-content/uploads/2024/01/Fediverse-3D_logo-V1-denoised.jpg",
      "mediaType": "image/jpeg",
      "name": "Banner"
    }
  ],
...

Using the type Document works fine.

...
"attachment": [
    {
      "type": "Document",
      "url": "http://wp.lan/wp-content/uploads/2024/01/Fediverse-3D_logo-V1-denoised.jpg",
      "mediaType": "image/jpeg",
      "name": "Banner"
    }
  ],
...

Suggestion: Maybe also just use the first attachment of type Image as the banner image when no attachment with name "Banner" is defined?