MimeMagic is a library to detect the mime type of a file by extension or by content.
require 'mimemagic'
MimeMagic.by_extension('html').text?
MimeMagic.by_extension('.html').child_of? 'text/plain'
MimeMagic.by_path('filename.txt')
MimeMagic.by_magic(File.open('test.html'))
# etc...
http://rdoc.info/github/minad/mimemagic/frames/file/README
Daniel Mendler