Uploads a new file and links it to a leasing contract.

This endpoint creates an upload associated with the specified leasing contract
and its accountable (asset or asset group). The upload is automatically placed
in a directory named after the translated document nature (e.g., "Move-in inventory report").

The file itself should be provided in the 'file' parameter as multipart/form-data,
while metadata is included in the 'upload' object.

Automatic behavior:

• The upload is linked to both the leasing contract and its accountable (asset/asset group)
• A directory is automatically created (or reused) based on the document nature
• The upload becomes visible in the leasing contract's upload list and the customer view