Apologies @andreas.lennartz - your message slipped through the .net
There’s no need to prepare or convert IFC files. The upload to the API endpoint is all that is required.
Looking at your sample code, the endpoint is different.
I posted about the methods for binary uploads in answer to someone asking about attachments: API post comments with attached files - #7 by jonathon
The upload endpoint has a listener that will detect IFC as the upload format (crudely by extension), and the parser will progress from there to make a model with the uploaded file’s name.