Fix parsing of Content-Disposition headers (#4763)

* Fix parsing of Content-Disposition headers

TIL: filenames in content-dispostion headers can contain semicolons, and aren't
%-encoded.

* fix python2 incompatibility

* Fix docstrings

tests/rest/media/v1/test_base.py

@@ -0,0 +1,45 @@
+# -*- coding: utf-8 -*-
+# Copyright 2019 New Vector Ltd
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from synapse.rest.media.v1._base import get_filename_from_headers
+
+from tests import unittest
+
+
+class GetFileNameFromHeadersTests(unittest.TestCase):
+    # input -> expected result
+    TEST_CASES = {
+        b"inline; filename=abc.txt": u"abc.txt",
+        b'inline; filename="azerty"': u"azerty",
+        b'inline; filename="aze%20rty"': u"aze%20rty",
+        b'inline; filename="aze\"rty"': u'aze"rty',
+        b'inline; filename="azer;ty"': u"azer;ty",
+
+        b"inline; filename*=utf-8''foo%C2%A3bar": u"foo£bar",
+    }
+
+    def tests(self):
+        for hdr, expected in self.TEST_CASES.items():
+            res = get_filename_from_headers(
+                {
+                    b'Content-Disposition': [hdr],
+                },
+            )
+            self.assertEqual(
+                res, expected,
+                "expected output for %s to be %s but was %s" % (
+                    hdr, expected, res,
+                )
+            )