How to get the file extension from a url?

Use File.extname

File.extname("test.rb")         #=> ".rb"
File.extname("a/b/d/test.rb")   #=> ".rb"
File.extname("test")            #=> ""
File.extname(".profile")        #=> ""

To format the string

"http://www.example.com/%s.%s" % [filename, extension]