Set the content type in CreateMultipartUpload

https://redmine.teklia.com/issues/11417

The arkindex.upload.serializers.MultipartUploadCreateSerializer should add a ContentType to the parameters sent to initiate_multipart_upload.

This ContentType should be set to application/zstd when uploading a ModelVersion, and to DataFile.content_type when uploading a DataFile.

For other object types (which we don't have yet, but could have later), the parameter should not be set.

The API documentation for CreateMultipartUpload should document that for a ModelVersion, a .tar.zst archive is always expected, and for a DataFile a file with the content type set in DataFile is expected.