This option should contain a comma separated list of options you can override. These options can then be changed specifically in various directories by adding them to a mc_access file. The default value of this option is "*" which tells TinyMCE to allow override of all options.
Example of usage in the config.php file
$mcImageManagerConfig['preview.allow_override'] = "urlsuffix";
Example of usage in the Web.Config file
<add key="preview.allow_override" value="urlsuffix" />
Example of usage in a mc_access file
# Override something
# Allow override