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?

未指出任何儲存庫檔案或測試。先參考連結的 Flask 範例和 Apollo 上傳伺服器，確定 flask-graphql 是否支援 multipart 檔案上傳；在估算實作工作量之前，定義預期的 GraphQL 介面和不使用瀏覽器的測試方法。

由索引模型根據 Issue 內容生成。