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require 'open-uri'
require 'uri'

html = open('http://stackoverflow.com/questions/8722693/how-to-get-all-image-pdf-and-other-files-links-from-a-web-page/8724632#8724632').read
puts URI.extract(html).select{ |l| l[/\.(?:gif|png|jpe?g)\b/]}