When allowing an extension file:/// URL access, grant its process access to that scheme.

chrome/common/extensions/api/extension_api.json

 [
   {
     "namespace": "extension",
     "types": [
       {
         "id": "MessageSender",
         "type": "object",
         "description": "An object containing information about the script context that sent a message or request.",
         "properties": {
           "tab": {"$ref": "Tab", "optional": true, "description":"This property will <b>only</b> be present when the connection was opened from a tab or content script."},
@@ skipping 6311 matching lines @@
                     "optional": true,
                     "description": "Details on the test server used to mock network responses.  Will be set only if test calls ExtensionApiTest::StartTestServer().",
                     "properties": {
                       "port": {
                         "type": "integer",
                         "description": "The port on which the test server is listening.",
                         "minimum": 1024,
                         "maximum": 65535
                       }
                     }
+                  },
+                  "testDataDirectory": {
+                    "type": "string",
+                    "description": "file:/// URL for the API test data directory."
                   }
                 }
               }
             ]
           }
         ]
       },
       {
         "name": "notifyFail",
         "unprivileged": true,
@@ skipping 2866 matching lines @@
                   }
                 }
               }
             ]
           }
         ]
       }
     ]
   }
 ]