[Python] Content-type is different when sending file to FastAPI backend using cURL than Python...

I have API which accepts JSON data and file (png).

This is my fastapi code:

@app.post("/api/")
async def dummy_file_return(metadata=Body(...),file=File(...)):
print("content_type is")
print(file.content_type)


When accesing with curl.

$curl -X POST -F file=@0_for_django_testcase.png -F metadata='{"meta":"test"}' localhost:8008/api/


server log shows,

content_type is
image/png
​

I can see content_type is guessed automatically and image/png is set.

Then, I tried the same thing by requests of python.

response = requests.post(
url,
data={"metadata":json.dumps({"meta":"test")},
files = {
"file": open('0_for_django_testcase.png','rb')
},
)


console shows

content_type is
​

content_type is empty and nothing appears.

Why this difference occurs?

In either way file upload is successed, however there is difference for content_type.

I don't set any header for curl though, -F flag secretly send some headers?

Another trial, I tested these patterns with headers, but both return <Response [400]> error.

response = requests.post(
url,
data={"metadata":json.dumps({"meta":"test")},
files = {
"file": open('0_for_django_testcase.png','rb')
},
headers={
"Content-Type":"image/png"
}
)


response = requests.post(
url,
data={"metadata":json.dumps({"meta":"test")},
files = {
"file": open('0_for_django_testcase.png','rb')
},
headers={
"Content-Type":"multipart/form-data"
}
)


Any help appreciated.

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