This option should contain a comma separated list of valid extensions all other file extensions will be denied. By default some image extensions are allowed.

Example of usage in the config.php file

$mcImageManagerConfig['upload.extensions'] = "gif,jpg,png,bmp"; 

Example of usage in the Web.Config file

<add key="upload.extensions" value="gif,jpg,png,bmp" />

Example of usage in a mc_access file