diff --git a/src/allmydata/webish.py b/src/allmydata/webish.py
index c3965645a..2d9ef25ff 100644
--- a/src/allmydata/webish.py
+++ b/src/allmydata/webish.py
@@ -44,6 +44,42 @@ from .web.storage_plugins import (
     StoragePlugins,
 )
 
+
+if PY2:
+    FileUploadFieldStorage = FieldStorage
+else:
+    class FileUploadFieldStorage(FieldStorage):
+        """
+        Do terrible things to ensure files are still bytes.
+
+        On Python 2, uploaded files were always bytes.  On Python 3, there's a
+        heuristic: if the filename is set on a field, it's assumed to be a file
+        upload and therefore bytes.  If no filename is set, it's Unicode.
+
+        Unfortunately, we always want it to be bytes, and Tahoe-LAFS also
+        enables setting the filename not via the MIME filename, but via a
+        separate field called "name".
+
+        Thus we need to do this ridiculous workaround.
+
+        Source for idea:
+        https://mail.python.org/pipermail/python-dev/2017-February/147402.html
+        """
+        @property
+        def filename(self):
+            if self.name == "file" and not self._mime_filename:
+                # We use the file field to upload files, see directory.py's
+                # _POST_upload. Lack of _mime_filename means we need to trick
+                # FieldStorage into thinking there is a filename so it'll
+                # return bytes.
+                return "unknown-filename"
+            return self._mime_filename
+
+        @filename.setter
+        def filename(self, value):
+            self._mime_filename = value
+
+
 class TahoeLAFSRequest(Request, object):
     """
     ``TahoeLAFSRequest`` adds several features to a Twisted Web ``Request``
@@ -94,7 +130,8 @@ class TahoeLAFSRequest(Request, object):
                 headers['content-length'] = str(self.content.tell())
                 self.content.seek(0)
 
-            self.fields = FieldStorage(self.content, headers, environ={'REQUEST_METHOD': 'POST'})
+            self.fields = FileUploadFieldStorage(
+                self.content, headers, environ={'REQUEST_METHOD': 'POST'})
             self.content.seek(0)
 
         self._tahoeLAFSSecurityPolicy()