Add ability to specify custom validation method

Author: rastating

lib/wpxf/wordpress/shell_upload.rb

@@ -93,6 +93,11 @@ def run
     true
   end
 
+  # @return [Boolean] true if the result of the upload operation is valid.
+  def validate_upload_result
+    true
+  end
+
   # Execute the payload at the specified address.
   # @param payload_url [String] the payload URL to access.
   def execute_payload(payload_url)
@@ -123,6 +128,6 @@ def upload_payload(builder)
       return false
     end
 
-    true
+    validate_upload_result
   end
 end