monkeypatch_content_type.rb

# Override Paperclip's content-type guessing support to add more MIME types.
# We actually have to monkey-patch File to do this, because Paperclip
# monkey-patches File to add #content_type (from module Paperclip::Upfile).
class File
  def content_type_with_extras
    type = (self.path.match(/\.(\w+)$/)[1] rescue "octet-stream").downcase
    case type
    when "pdf" then "application/pdf"
    # add more content types here
    else content_type_without_extras
    end
  end

  alias_method_chain :content_type, :extras
end