This rack middleware can be used to detect content type of attachment

fix_content_type.rb

require 'shellwords'

class FixAttachmentContentType
  def initialize(app)
    @app = app
  end

  def call(env)
    if form_hash = env["rack.request.form_hash"]
      fix_tempfiles(form_hash)
    end
    @app.call(env)
  end

  # this method performs recursive traversal of hash, finds sub-hashes
  # with :tempfile key and fix content type via file utility
  #
  def fix_tempfiles(form_hash)
    form_hash.each do |key, val|
      if val.is_a?(Hash)
        fix_tempfiles(val)
        if val[:tempfile]
          val[:type] = `file -b -i #{val[:tempfile].path.shellescape}`.chomp.gsub(/;.*$/, '')
        end
      end
    end
  end
end