Handle file upload in spawner form

Allow files to be uploaded in the spawner form.

jupyterhub/tests/test_pages.py

@@ -119,3 +119,31 @@ def test_spawn_form(app, io_loop):
             'notspecified': 5,
         }
 
+def test_spawn_form_with_file(app, io_loop):
+    with mock.patch.dict(app.users.settings, {'spawner_class': FormSpawner}):
+        base_url = ujoin(app.proxy.public_server.host, app.hub.server.base_url)
+        cookies = app.login_user('jones')
+        orm_u = orm.User.find(app.db, 'jones')
+        u = app.users[orm_u]
+        io_loop.run_sync(u.stop)
+
+        r = requests.post(ujoin(base_url, 'spawn'),
+                          cookies=cookies,
+                          data={
+                              'bounds': ['-1', '1'],
+                              'energy': '511keV',
+                          },
+                          files={'hello': ('hello.txt', b'hello world\n')}
+                      )
+        r.raise_for_status()
+        print(u.spawner)
+        print(u.spawner.user_options)
+        assert u.spawner.user_options == {
+            'energy': '511keV',
+            'bounds': [-1, 1],
+            'notspecified': 5,
+            'hello': {'filename': 'hello.txt',
+                      'body': b'hello world\n',
+                      'content_type': 'application/unknown'},
+        }
+