#3667: Update the s3 post functionality to better support success_action_redirect (#3668)

* Update the s3 post functionality to better support success_action_redirect

- Add the bucket/key values to the redirect url like s3 does, which
supports code that relies on the key value being there on the
redirect.
- Add support for replacing ${filename} in the key value with the actual
filename from the form upload.

See Issue #3667

* Update s3 tests for changed success_action_redirect behavior

- Adds a new test called test_s3_server_post_to_bucket_redirect that
tests both the ${filename} replacement and the key/value addition to the
redirect query args
- Updated the test_creating_presigned_post checks to handle the
key/value additions to the redirect url.

* Fix test updates to work with python2.7

- remove f-string usage
- fix urllib.parse imports to use six

Co-authored-by: Wynn Wilkes <wynn@leading2lean.com>

moto/s3/responses.py

@@ -1,5 +1,6 @@
 from __future__ import unicode_literals
 
+import os
 import re
 import sys
 
@@ -11,7 +12,14 @@ from moto.core.utils import (
     py2_strip_unicode_keys,
     unix_time_millis,
 )
-from six.moves.urllib.parse import parse_qs, urlparse, unquote, parse_qsl
+from six.moves.urllib.parse import (
+    parse_qs,
+    parse_qsl,
+    urlparse,
+    unquote,
+    urlencode,
+    urlunparse,
+)
 
 import xmltodict
 
@@ -859,10 +867,28 @@ class ResponseObject(_TemplateEnvironmentMixin, ActionAuthenticatorMixin):
         if "file" in form:
             f = form["file"]
         else:
-            f = request.files["file"].stream.read()
+            fobj = request.files["file"]
+            f = fobj.stream.read()
+            key = key.replace("${filename}", os.path.basename(fobj.filename))
 
         if "success_action_redirect" in form:
-            response_headers["Location"] = form["success_action_redirect"]
+            redirect = form["success_action_redirect"]
+            parts = urlparse(redirect)
+            queryargs = parse_qs(parts.query)
+            queryargs["key"] = key
+            queryargs["bucket"] = bucket_name
+            redirect_queryargs = urlencode(queryargs, doseq=True)
+            newparts = (
+                parts.scheme,
+                parts.netloc,
+                parts.path,
+                parts.params,
+                redirect_queryargs,
+                parts.fragment,
+            )
+            fixed_redirect = urlunparse(newparts)
+
+            response_headers["Location"] = fixed_redirect
 
         if "success_action_status" in form:
             status_code = form["success_action_status"]