Add `mime-types` gem into the gemspec as a de

Closes #507

paperclip.gemspec

@@ -27,6 +27,7 @@ spec = Gem::Specification.new do |s|
   s.add_dependency 'activerecord', '>=2.3.0'
   s.add_dependency 'activesupport', '>=2.3.2'
   s.add_dependency 'cocaine', '>=0.0.2'
+  s.add_dependency 'mime-types'
   s.add_development_dependency 'shoulda'
   s.add_development_dependency 'appraisal'
   s.add_development_dependency 'mocha'