Update file_uploads.md

Author: ThibBal

docs/file_uploads.md

@@ -43,6 +43,12 @@ class MyController
 }
 ```
 
-Of course, you need to use a GraphQL client that is compatible with multipart requests.
+Of course, you need to use a GraphQL client that is compatible with multipart requests. The GraphQL client must send the file using the Upload type.
+
+```
+    mutation upload($file: Upload!) {
+        upload(file: $file)
+    }
+```
 
 See [jaydenseric/graphql-multipart-request-spec](https://github.com/jaydenseric/graphql-multipart-request-spec#client) for a list of compatible clients.