This options gives you the ability to change the error message that is displayed to the user if the regexp patterns denies the file name.
This can be very useful if the user needs to understand how the regexp patterns works and why they denied the name.
Example of usage in the config.php file
$mcImageManagerConfig['upload.invalid_file_name_msg'] = "Error: The name of the file is invalid.";
Example of usage in the Web.Config file
<add key="upload.invalid_file_name_msg" value="Error: The name of the file is invalid." />
Example of usage in a mc_access file
upload.invalid_file_name_msg=Error: The name of the file is invalid.