From FCKeditor Docs

To create a new resource type you should place the section below in your config.cfm file. Note that the options you can use are discribed in the Built-in resource types section.

Example: You want to create a resource type MSdocs which will only have files with the doc,ppt,xls extensions and be maximum 2 megabites large.

config.resourceType[1] = structNew();
config.resourceType[1].name = 'MSdocs';
config.resourceType[1].url = config.baseUrl & 'MSdocs';
config.resourceType[1].directory = config.baseDir & 'MSdocs';
config.resourceType[1].maxSize = "2M";
config.resourceType[1].allowedExtensions = 'doc,ppt,xls';
config.resourceType[1].deniedExtensions = '';

When loading CKFinder, the "type" querystring parameter can be used to display a specific type only. If "type" is omitted in the URL, the "DefaultResourceTypes" settings is used (may contain the resource type names separated by a comma).
If left empty, all types are loaded. If you want to limit the CKFinder to use only MSdocs resource type, use the following code:

config.defaultResourceTypes = 'MSdocs';