curl --request POST \
  --url https://api.scalev.com/v3/stores/{store_id}/public/orders/{secret_slug}/transfer-proof-upload \
  --header 'Content-Type: application/json' \
  --header 'X-Scalev-Storefront-Api-Key: <x-scalev-storefront-api-key>' \
  --data '
{
  "filename": "transfer-proof.png",
  "content_type": "image/jpeg",
  "content_length": 5242880
}
'

{
  "key": "<string>",
  "filename": "<string>",
  "content_type": "<string>",
  "size_in_bytes": 123,
  "file_url": "<string>",
  "upload_url": "<string>"
}