sending with attachments not working

[aerickson]

I get an error when attempting to send attachments when running from 7bb76b3 (head of the develop branch).

Sending with attachments does work at dd15a0a.

I'm running Python 3.6.4. Here's the error I'm getting:

(my code is above this point in the stack)
...
08:45:44 web.1  |   File "/Users/aje/git/inqqqq/venv/src/flask-mailgun3/flask_mailgun/core.py", line 51, in send
08:45:44 web.1  |     return self.mailgun_api.send(message)
08:45:44 web.1  |   File "/Users/aje/git/inqqqq/venv/src/flask-mailgun3/flask_mailgun/api.py", line 80, in send
08:45:44 web.1  |     files=files)
08:45:44 web.1  |   File "/Users/aje/git/inqqqq/venv/lib/python3.6/site-packages/requests/api.py", line 112, in post
08:45:44 web.1  |     return request('post', url, data=data, json=json, **kwargs)
08:45:44 web.1  |   File "/Users/aje/git/inqqqq/venv/lib/python3.6/site-packages/requests/api.py", line 58, in request
08:45:44 web.1  |     return session.request(method=method, url=url, **kwargs)
08:45:44 web.1  |   File "/Users/aje/git/inqqqq/venv/lib/python3.6/site-packages/requests/sessions.py", line 494, in request
08:45:44 web.1  |     prep = self.prepare_request(req)
08:45:44 web.1  |   File "/Users/aje/git/inqqqq/venv/lib/python3.6/site-packages/requests/sessions.py", line 437, in prepare_request
08:45:44 web.1  |     hooks=merge_hooks(request.hooks, self.hooks),
08:45:44 web.1  |   File "/Users/aje/git/inqqqq/venv/lib/python3.6/site-packages/requests/models.py", line 308, in prepare
08:45:44 web.1  |     self.prepare_body(data, files, json)
08:45:44 web.1  |   File "/Users/aje/git/inqqqq/venv/lib/python3.6/site-packages/requests/models.py", line 496, in prepare_body
08:45:44 web.1  |     (body, content_type) = self._encode_files(files, data)
08:45:44 web.1  |   File "/Users/aje/git/inqqqq/venv/lib/python3.6/site-packages/requests/models.py", line 162, in _encode_files
08:45:44 web.1  |     rf.make_multipart(content_type=ft)
08:45:44 web.1  |   File "/Users/aje/git/inqqqq/venv/lib/python3.6/site-packages/urllib3/fields.py", line 174, in make_multipart
08:45:44 web.1  |     (('name', self._name), ('filename', self._filename))
08:45:44 web.1  |   File "/Users/aje/git/inqqqq/venv/lib/python3.6/site-packages/urllib3/fields.py", line 134, in _render_parts
08:45:44 web.1  |     parts.append(self._render_part(name, value))
08:45:44 web.1  |   File "/Users/aje/git/inqqqq/venv/lib/python3.6/site-packages/urllib3/fields.py", line 114, in _render_part
08:45:44 web.1  |     return format_header_param(name, value)
08:45:44 web.1  |   File "/Users/aje/git/inqqqq/venv/lib/python3.6/site-packages/urllib3/fields.py", line 35, in format_header_param
08:45:44 web.1  |     if not any(ch in value for ch in '"\\\r\n'):
08:45:44 web.1  |   File "/Users/aje/git/inqqqq/venv/lib/python3.6/site-packages/urllib3/fields.py", line 35, in <genexpr>
08:45:44 web.1  |     if not any(ch in value for ch in '"\\\r\n'):
08:45:44 web.1  | TypeError: a bytes-like object is required, not 'str'