
Hello!
So I solved the problem. I changed the accepted format by adding "req.setHeader('accept', 'application/json');" in case anyone needs it.
Full code:
Http h = new Http();
HttpRequest req = new HttpRequest();
req.setEndpoint('callout:ClarizenAPI/files/getUploadUrl');
req.setMethod('GET');
req.setHeader('accept', 'application/json');
req.setHeader('Authorization', sessionId);
HttpResponse response = h.send(req);
Thank you Roland for helping me!