Uploading file with extra payload in variables doesn't seem working
After days of investigation (supertest, apollo, graphql-upload, ...) I finally could find the origin of my issue: when I upload a file, using extra payload directly in the query, it works:

<img width="702" alt="Screenshot 2022-08-15 at 15 31 43" src="https://user-images.githubusercontent.com/1402505/184823581-edf4da4a-2b70-423f-8ac6-253d0ae67078.png">

If I add the extra payload in variables, it does'nt work (I replaced here `input` from an object to a string for simplicity of debugging):

<img width="980" alt="Screenshot 2022-08-16 at 09 35 30" src="https://user-images.githubusercontent.com/1402505/184823807-d524b762-c807-46f4-b4fd-799985eb04d7.png">

Operations: `{ "query": "mutation a($file: Upload!, $input: String!) { uploadFile(file: $file, input: $input) }" }`
Map: `{ "0": ["variables.file"], "1": ["variables.input"] }`

I found a lot of examples / tuto / forum topics explaininig how to deal with supertest, like [this one](https://github.com/jaydenseric/graphql-upload/issues/125), very helpful, but then I realiseed nobody was trying to add more than 1 param (the file to upload).
Is there really a bug or I misunderstand something?
I also tried to use FormData (not compatible anymore with supertest), to inject the variables directly in `operations`, to try with postman, curl, supertest, nodejs, ...