import cgi
from io import BytesIO


body = BytesIO(b"""--MyBoundary
Content-Disposition: form-data; name="my-arg"; filename="foo"

Test

--MyBoundary--
""")
headers = {
    "REQUEST_METHOD": "POST",
    "CONTENT_TYPE": "multipart/form-data; boundary=MyBoundary",
    "wsgi.input": body,
}
fields = cgi.FieldStorage(body, environ=headers)

assert len(fields["my-arg"].file.read()) == 5