Searching through a loop of filenames and looking for a particular file extension type.

I have the following code and I'm trying to make sure it adds only to a listbox if it finds the .PLT file extension.  For some reason its still adding all the files it finds.  I would also like to change whats below to account for more than one file type extensions.  For example, I may want to check for .PDF and .doc files that are in a collection.  Instead of running the collection and then doing the file search what is the best method to keep the searching from only performing once?

foreach (string fileName in fileEntries)
            {
                // do something with fileName
   
                Directions = Path.GetFileName(fileName);

                Console.WriteLine(Directions.LastIndexOf(".plt"));

                if (Directions.LastIndexOf(".plt") !=0 || Directions.LastIndexOf(".PLT") !=0)
                {
                    //theName = Path.GetFileName((fileName));
                    //theName = (fileName);
                    lstFrom.Items.Add(Directions);
                }
             
            }Start Free Trial