Hey @LOIJon - welcome to the community!
That’s a bit odd…I just tried running this in Postman and the query worked:
mutation ($file: File!) {add_file_to_column (file: $file, item_id: xxxx, column_id: "files") {id}}
You’ll want to make sure the ‘variables’ key is set to file, and that you are uploading as ‘form-data’:
-Daniel