Hi,
Firstly you should not include dots before extensions in the property:
ExcludedExtensionsForPreview = "pdf, xls, doc, docx"
This way the property would work as expected but you would still need to list all extensions (no match-all setting here).
For disabling all previewing, instead you should deny Preview permission for your root folder:
var rootFolder = new FileManagerRootFolder
{
Name = "My root folder",
Location = "c:\root\folder"
};
rootFolder.AccessControls.Add(new FileManagerAccessControl
{
Path = @"\",
AllowedPermissions = FileManagerPermissions.Full,
DeniedPermissions = FileManagerPermissions.Preview
});
fileManager.RootFolders.Add(rootFolder);