You can actually use a list of tuples in the file param and eliminate the need for Multidict. Here's what it would look like:
import requests
print requests.post("https://api.mailgun.net/v2/samples.mailgun.org/messages",
auth=("api", "key-3ax6xnjp29jd6fds4gc373sgvjxteol0"),
files=[("attachment", open("files/test.jpg")),
("attachment", open("files/test.txt"))],
data={"from": "Excited User <me@samples.mailgun.org>",
"to": "foo@example.com",
"cc": "baz@example.com",
"bcc": "bar@example.com",
"subject": "Hello",
"text": "Testing some Mailgun awesomness!",
"html": "<html>HTML version of the body</html>"})
Disclaimer: I work for Mailgun!