File Upload

I'm working on an [example repo](https://github.com/savovs/flask-graphene-hello) and was wondering, what would be the simplest way to implement that? This is what I tried without GraphQL:

```python
@app.route('/upload', methods=['POST'])
def uploadFile():
for key in request.files:
file = request.files[key]
file.save(os.path.join(UPLOAD_FOLDER, file.filename))

return 'uploaded'
```

[This is what](https://github.com/jaydenseric/apollo-upload-server) I used in Node.js, would love to do something like that for Flask in Python.

If it's not supported by flask-graphql I'd be happy to work on a pull request. Also, how would you test this without a browser, using Graphiql, Postman or something like that?

Nenhum arquivo ou teste do repositório é mencionado. Comece determinando se flask-graphql oferece suporte a uploads de arquivos multipart, usando como referências o exemplo de Flask vinculado e o servidor de upload Apollo; defina a interface GraphQL esperada e uma abordagem de testes sem navegador antes de estimar o trabalho de implementação.

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