Mint a signed multipart upload URL for a custom thumbnail

Returns a multipart upload URL set for a user-supplied custom thumbnail image. Once the upload is finalized via `POST /assets/{assetId}/custom-thumbnail/complete-upload`, background processing generates the sized `customThumbnail` files and registers them on the asset. Side effect: any prior active `customThumbnailOriginal` / `customThumbnail` files on the asset are set to `inactive` before the URL is issued so the new upload replaces them cleanly. If the upload is abandoned, the asset's auto-generated `thumbnail` files remain and should be displayed instead. Custom thumbnails are only supported for `video` and `audio` assets.

curl -X POST "https://example.com/assets/497f6eca-6276-4993-bfeb-53cbbbba6f08/custom-thumbnail/upload-url" \  -H "Content-Type: application/json" \  -d '{    "fileName": "cover.jpg",    "mimeType": "image/jpeg",    "sizeInMB": 1.2  }'

{  "fileName": "string",  "assetId": "9179b887-04ef-4ce5-ab3a-b5bbd39ea3c8",  "uploadId": "string",  "key": "string",  "urls": [    "string"  ],  "mimeType": "string",  "expires": 0,  "status": "success"}