How to pass data from client to middleware in uploadthing?

I have the button from uploadthing and i try to pass data by headers to the middleware but i get noting.

the button:

      <UploadButton
        endpoint="imageUploader"
        headers={{
          "X-Custom-Header": "custom-value22222222", // data i test to submit
          data: "test",
        }}

the middleware:

  .middleware(async ({ req }) => {
console.log('middleware to get test data header:', req) // can't get above headers
return { userId: user.id};
})

any ideas? their docs are not fully complete