diff --git a/newsfragments/3577.minor b/newsfragments/3577.minor
new file mode 100644
index 000000000..e69de29bb
diff --git a/src/allmydata/test/web/test_webish.py b/src/allmydata/test/web/test_webish.py
index e680acd04..12a04a6eb 100644
--- a/src/allmydata/test/web/test_webish.py
+++ b/src/allmydata/test/web/test_webish.py
@@ -1,6 +1,16 @@
 """
 Tests for ``allmydata.webish``.
+
+Ported to Python 3.
 """
+from __future__ import absolute_import
+from __future__ import division
+from __future__ import print_function
+from __future__ import unicode_literals
+
+from future.utils import PY2
+if PY2:
+    from future.builtins import filter, map, zip, ascii, chr, hex, input, next, oct, open, pow, round, super, bytes, dict, list, object, range, str, max, min  # noqa: F401
 
 from uuid import (
     uuid4,
@@ -96,7 +106,7 @@ class TahoeLAFSRequestTests(SyncTestCase):
         ])
         self._fields_test(
             b"POST",
-            {b"content-type": b"multipart/form-data; boundary={}".format(boundary)},
+            {b"content-type": b"multipart/form-data; boundary=" + bytes(boundary, 'ascii')},
             form_data.encode("ascii"),
             AfterPreprocessing(
                 lambda fs: {
@@ -105,8 +115,8 @@ class TahoeLAFSRequestTests(SyncTestCase):
                     in fs.keys()
                 },
                 Equals({
-                    b"foo": b"bar",
-                    b"baz": b"some file contents",
+                    "foo": "bar",
+                    "baz": b"some file contents",
                 }),
             ),
         )
diff --git a/src/allmydata/util/_python3.py b/src/allmydata/util/_python3.py
index a7b77001a..b2bc47153 100644
--- a/src/allmydata/util/_python3.py
+++ b/src/allmydata/util/_python3.py
@@ -110,6 +110,7 @@ PORTED_MODULES = [
     "allmydata.util.spans",
     "allmydata.util.statistics",
     "allmydata.util.time_format",
+    "allmydata.webish",
 ]
 
 PORTED_TEST_MODULES = [
@@ -179,6 +180,7 @@ PORTED_TEST_MODULES = [
     "allmydata.test.test_upload",
     "allmydata.test.test_uri",
     "allmydata.test.test_util",
+    "allmydata.test.test_webish",
     "allmydata.test.web.test_common",
     "allmydata.test.web.test_grid",
     "allmydata.test.web.test_util",
diff --git a/src/allmydata/webish.py b/src/allmydata/webish.py
index f32f56714..b3b819b6a 100644
--- a/src/allmydata/webish.py
+++ b/src/allmydata/webish.py
@@ -1,3 +1,15 @@
+"""
+Ported to Python 3.
+"""
+from __future__ import absolute_import
+from __future__ import division
+from __future__ import print_function
+from __future__ import unicode_literals
+
+from future.utils import PY2
+if PY2:
+    from future.builtins import filter, map, zip, ascii, chr, hex, input, next, oct, open, pow, round, super, bytes, dict, list, object, range, str, max, min  # noqa: F401
+
 from six import ensure_str
 
 import re, time, tempfile
@@ -65,18 +77,24 @@ class TahoeLAFSRequest(Request, object):
             self.path, argstring = x
             self.args = parse_qs(argstring, 1)
 
-        if self.method == 'POST':
+        if self.method == b'POST':
             # We use FieldStorage here because it performs better than
             # cgi.parse_multipart(self.content, pdict) which is what
             # twisted.web.http.Request uses.
-            self.fields = FieldStorage(
-                self.content,
-                {
-                    name.lower(): value[-1]
-                    for (name, value)
-                    in self.requestHeaders.getAllRawHeaders()
-                },
-                environ={'REQUEST_METHOD': 'POST'})
+
+            headers = {
+                ensure_str(name.lower()): ensure_str(value[-1])
+                for (name, value)
+                in self.requestHeaders.getAllRawHeaders()
+            }
+
+            if 'content-length' not in headers:
+                # Python 3's cgi module would really, really like us to set
+                # Content-Length. This seems likely to shoot performance in
+                # the foot.
+                headers['content-length'] = len(self.content.getvalue())
+
+            self.fields = FieldStorage(self.content, headers, environ={'REQUEST_METHOD': 'POST'})
             self.content.seek(0)
 
         self._tahoeLAFSSecurityPolicy()