Urllib3 POST request with attachment in python - python-3.x

Question

I wish to make a post request to add an attachment utilising urllib3 in python without success. I have confirmed the API itself is working in postman but cannot work out how to convert this request to python. Appreciating I'm mixing object types I just don't know how to avoid it.
Python code:
import urllib3
import json
api_key = "secret_key"
header = {"X-API-KEY": api_key, "ACCEPT": "application/json", "content-type": "multipart/form-data"}
url = "https://secret_url.com/api/"
http = urllib3.PoolManager()
with open("invoice.html", 'rb') as f:
file_data = f.read()
payload = {
"attchment": {
"file": file_data
}
}
payload = json.dumps(payload)
r = http.request('post', url, headers = header, fields = payload)
print(r.status)
print(r.data)
Postman - which works and properly sends file-name through also (I'm guessing it splits the bytes and filename up?)
Edit: I've also tried the requests library as I'm more familiar with this (but can't use it as the script will be running in AWS lambda). Removing the attachment element form the dict allows it to run but the API endpoint gives 401 presumably because it's missing the "attachement" part to the data structure as per postman below... but when I put this in I get runtime errors.
r = requests.post(url, headers = header, files={"file": open("invoice.html", 'rb')})

Answer 1

For anyone who stumbles upon this from Dr google a few points:
I was completely mis-interpreting the structure of the element. It's actually a string "attachment[file]" not a dict like object.
Postman has the ability to output python code in urllib/request syntax albeit not 100% what I was after. Note: the chrome version (depreciated) outputs gibberish code that only half works so the client version should be used. A short bit of work below shows it working as expected:
http = urllib3.PoolManager()
with open("invoice.html", "rb") as f:
file = f.read()
payload={
'attachment[file]':('invoice.html',file,'text/html')
}
r = http.request('post', url, headers = header, fields = payload)