[Python] python set-up boundary for POST using multipart/form-data with requests

I want to send a file using requests but the server works with a fixed boundary set at *****. I'm only able to send a file but the requests module creates a random boundary. How do I overwrite it?

import requests

url='http://xxx.xxx.com/uploadfile.php'
fichier= {'uploadedfile'pen('1103290736_2016_03_23_13_32_55.zip','rb')}
headers2={'Connection':'Keep-Alive','User-Agent':'Dalvik/1.6.0 (Linux; U; Android 4.4.2; S503+ Build/KOT49H)','Accept-Encoding':'gzip'}
session= requests.Session()
session.post(url,headers=headers2,files=fichier)
session.close()

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