Hey @rose!
Do you have a working call to your GraphQL API from something like Postman that we might use for reference?
As a note - when you select the "Form Data" type Retool will automatically set the content-type header with multipart/form-data and the boundary.
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.