POST Policy, multiple fixes: AccessDenied with unmet conditions, ${filename} in Key, missing filename in multipart (#2304)

* Unsatisfied conditions will return AccessDenied instead of MissingFields

* Require form-field `file` in POST policy and make `filename` an optional attribute

* S3 feature: Replace  in Key by filename attribute passed in multipart

bucket-handlers.go

@@ -347,7 +347,7 @@ func (api objectAPIHandlers) PostPolicyBucketHandler(w http.ResponseWriter, r *h
 		return
 	}
 
-	fileBody, formValues, err := extractHTTPFormValues(reader)
+	fileBody, fileName, formValues, err := extractHTTPFormValues(reader)
 	if err != nil {
 		errorIf(err, "Unable to parse form values.")
 		writeErrorResponse(w, r, ErrMalformedPOSTRequest, r.URL.Path)
@@ -357,6 +357,12 @@ func (api objectAPIHandlers) PostPolicyBucketHandler(w http.ResponseWriter, r *h
 	formValues["Bucket"] = bucket
 	object := formValues["Key"]
 
+	if fileName != "" && strings.Contains(object, "${filename}") {
+		// S3 feature to replace ${filename} found in Key form field
+		// by the filename attribute passed in multipart
+		object = strings.Replace(object, "${filename}", fileName, -1)
+	}
+
 	// Verify policy signature.
 	apiErr := doesPolicySignatureMatch(formValues)
 	if apiErr != ErrNone {