I was trying to upload an image file to my Graphql API, and since multipart doesn't work with the GraphQL resource, I've tried to do this using the Rest API resource instead and making a POST request to my Graphql API.
You might try removing the header you've set in the query settings and see if that works.
It's also on the dev team's radar to include support for uploading files with the GraphQL integration, I'll let you know here when the feature is included.