CORs error POSTing to Fusion webhook with Multipart Form Data

We have several webhooks which successfully receive HTML files via POST (using JQuery) to Fusion Webhooks. We are now developing a new scenario where we want to submit image / text / ics files to a Fusion webhook. No problem sending from Postman, so we know the 'structure' of the post is right, but as soon as we use JQuery we get the following error:

Access to XMLHttpRequest at 'https://app.workfrontfusion.com/wh/XXXXX' from origin 'https://XXX.XXX.com' has been blocked by CORS policy: Response to preflight request doesn't pass access control check: The 'Access-Control-Allow-Origin' header has a value 'https://app.workfrontfusion.com' that is not equal to the supplied origin.

I'm hoping there's someone out there who's run into this already and knows the solution - my suspicion is that it has something to do with the fact that it's Multipart form data - but the documentation is a little unclear on exact requirement / structure (at least to me) 😔 . And while we're clever people - we don't have any devs in the team who know this sort of stuff hence leaning on the community here.

Is there anyway to enable or allow cross-domain requests from defined domains in Fusion perhaps?