[Python] FastAPI - How to pass a dictionary/JSON data in HTTP multipart request?

I'm trying to make a POST request:

import requests


files = {'template': open('template.xlsx', 'rb')}
payload = {
'context': {
'OUT': 'csv',
'SHORT': 'short'
},
'filename': 'file.xlsx',
'content_type': 'application/excel'
}

r = requests.post('http://localhost:8000/render', files=files, data=payload)


to a FastAPI server:

from fastapi import FastAPI, UploadFile, Form
from pydantic import Json

app = FastAPI()


@app.post('/render')
def render(template: UploadFile, context: Json = Form(), filename: str = Form(...), content_type: str = Form(...)):
# processing
return "ok"


but I get this error (422 status code):

{"detail":[{"loc":["body","context"],"msg":"Invalid JSON","type":"value_error.json"}]}


As you can see, I'm trying to pass both a file and request body in the same request. I guess I could fix this if converted payload['context'] into JSON. But I'd like to fix this on server side.

How can I fix the error? Maybe convert some before params passed into view or something like this?

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