the \ escapes the .
without out it, the string means "any character followed by m, p, 3 and then the end of the line". With it it means ". followed by m, then p, then 4 then end of line. So without it, a path like "http://domain.com/winamp3" would match, when it is not likely to actually be an mp3 file, but a directory. Obviously things like mime types are relevant here as well, but at that level that's the difference.