Exclude files with a certain extension from list

string invalidExtension = (from p in new List<string> 

{ ".dll", ".pdb", ".suo", ".docstates", ".ico", ".bmp", ".vs10x", ".vspscc", ".vssscc", ".vsp", ".scc", ".user", ".ctl", ".datasource", ".vsmdi", ".testsettings", ".trx", ".dat" }
                            where p == files[i].Extension.ToLower()
                            select p).FirstOrDefault();
 
if (string.IsNullOrEmpty(invalidExtension))
{

//Means it has an invalid extension

}