fix: delete content-type header when is used upload middleware

fixes the issue described [here](#41 (comment))

src/middlewares/upload.js

@@ -37,6 +37,7 @@ export default function uploadMiddleware(): Middleware {
 
       req.fetchOpts.method = 'POST';
       req.fetchOpts.body = formData;
+      delete req.fetchOpts.headers['Content-Type'];
     }
 
     const res = await next(req);