If you want to watch right away, Netflix is testing the 4K streaming waters with select shows (House of Cards, Breaking Bad, The Blacklist) and films (Ghostbusters, The Smurfs 2) if you own a Samsung, Sony or LG 4K TV and have a 25Mbps internet connection.
A PRN file is a special type of file which contains instructions for a printer, it tells the printer what to print on the page and where as well as which paper tray to use, what the paper size is and a number of other controls.
We also have .BPN files (backup files), .TPL files (template files), .GSM files (object files), .LCF (library container files), plus a ton of ever more obscure file types, which we can talk about another time. See the full list of file types here.