ruby rookie needs help with mtime method of File

I was looking at an example of the "mtime" method for a file and when I ran
the example I received the following traceback

C:\barry\ruby>ruby dir3-glob.rb
backtick1.rb
Traceback (most recent call last):
        2: from dir3-glob.rb:3:in `<main>'
        1: from dir3-glob.rb:3:in `glob'
dir3-glob.rb:6:in `block in <main>': undefined method `mtime' for
"backtick1.rb":String (NoMethodError)

The source of my script is as follows
     1 #!C:\Ruby\bin\ruby.exe -w
     2
     3 Dir.glob("*.rb") {|file|
     4  # do something with the file here
     5  puts file
     6  mtime = file.mtime
     7  new_filename = "#{mtime.year}-#{mtime.month}-#{mtime.day}.wma"
     8  puts "Renaming #{filename} to #{new_filename} ..."
     9 }

What have I done wrong?

==================

Barry Kimelman
Winnipeg, Manitoba, Canada

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