Add ability to specify the file extension to use for the payload

Author: rastating

lib/wpxf/wordpress/shell_upload.rb

@@ -65,14 +65,19 @@ def upload_request_params
     nil
   end
 
+  # @return [String] the extension type to use when generating the payload name.
+  def payload_name_extension
+    'php'
+  end
+
   # Run the module.
   # @return [Boolean] true if successful.
   def run
     return false unless super
     return false unless before_upload
 
     emit_info 'Preparing payload...'
-    @payload_name = "#{Utility::Text.rand_alpha(payload_name_length)}.php"
+    @payload_name = "#{Utility::Text.rand_alpha(payload_name_length)}.#{payload_name_extension}"
     builder = payload_body_builder
     return false unless builder