diff --git a/newsfragments/3854.bugfix b/newsfragments/3854.bugfix
new file mode 100644
index 000000000..d12e174f9
--- /dev/null
+++ b/newsfragments/3854.bugfix
@@ -0,0 +1 @@
+Fixed regression on Python 3 where JSON HTTP POSTs failed to be processed.
\ No newline at end of file
diff --git a/src/allmydata/test/web/test_webish.py b/src/allmydata/test/web/test_webish.py
index 12a04a6eb..4a77d21ae 100644
--- a/src/allmydata/test/web/test_webish.py
+++ b/src/allmydata/test/web/test_webish.py
@@ -90,10 +90,11 @@ class TahoeLAFSRequestTests(SyncTestCase):
         """
         self._fields_test(b"GET", {}, b"", Equals(None))
 
-    def test_form_fields(self):
+    def test_form_fields_if_filename_set(self):
         """
         When a ``POST`` request is received, form fields are parsed into
-        ``TahoeLAFSRequest.fields``.
+        ``TahoeLAFSRequest.fields`` and the body is bytes (presuming ``filename``
+        is set).
         """
         form_data, boundary = multipart_formdata([
             [param(u"name", u"foo"),
@@ -121,6 +122,49 @@ class TahoeLAFSRequestTests(SyncTestCase):
             ),
         )
 
+    def test_form_fields_if_name_is_file(self):
+        """
+        When a ``POST`` request is received, form fields are parsed into
+        ``TahoeLAFSRequest.fields`` and the body is bytes when ``name``
+        is set to ``"file"``.
+        """
+        form_data, boundary = multipart_formdata([
+            [param(u"name", u"foo"),
+             body(u"bar"),
+            ],
+            [param(u"name", u"file"),
+             body(u"some file contents"),
+            ],
+        ])
+        self._fields_test(
+            b"POST",
+            {b"content-type": b"multipart/form-data; boundary=" + bytes(boundary, 'ascii')},
+            form_data.encode("ascii"),
+            AfterPreprocessing(
+                lambda fs: {
+                    k: fs.getvalue(k)
+                    for k
+                    in fs.keys()
+                },
+                Equals({
+                    "foo": "bar",
+                    "file": b"some file contents",
+                }),
+            ),
+        )
+
+    def test_form_fields_require_correct_mime_type(self):
+        """
+        The body of a ``POST`` is not parsed into fields if its mime type is
+        not ``multipart/form-data``.
+
+        Reproducer for https://tahoe-lafs.org/trac/tahoe-lafs/ticket/3854
+        """
+        data = u'{"lalala": "lolo"}'
+        data = data.encode("utf-8")
+        self._fields_test(b"POST", {"content-type": "application/json"},
+                          data, Equals(None))
+
 
 class TahoeLAFSSiteTests(SyncTestCase):
     """
diff --git a/src/allmydata/webish.py b/src/allmydata/webish.py
index 9b63a220c..519b3e1f0 100644
--- a/src/allmydata/webish.py
+++ b/src/allmydata/webish.py
@@ -114,7 +114,8 @@ class TahoeLAFSRequest(Request, object):
             self.path, argstring = x
             self.args = parse_qs(argstring, 1)
 
-        if self.method == b'POST':
+        content_type = (self.requestHeaders.getRawHeaders("content-type") or [""])[0]
+        if self.method == b'POST' and content_type.split(";")[0] in ("multipart/form-data", "application/x-www-form-urlencoded"):
             # We use FieldStorage here because it performs better than
             # cgi.parse_multipart(self.content, pdict) which is what
             # twisted.web.http.Request uses.