Uploads a new file with the specified metadata.

This endpoint requires a multipart/form-data request with both the file content
and metadata about the upload. The file itself should be provided in the 'file'
parameter, while metadata is included in the 'upload' object.

Files can be optionally associated with other entities in the system by providing
the resourceable_id and resourceable_type parameters. For example, to attach a file
to a leasing contract, you would set resourceable_type to 'LeasingContract' and
resourceable_id to the contract's ID.

Files can also be organized in directories by specifying a parent upload_directory_id.