Very quickly implemented MIME Type validator for CarrierWave. See also: https://gist.github.com/1009861 and https://github.com/jnicklas/carrierwave/wiki/How-to%3A-Validate-attachment-file-size

en.yml

en:
  errors:
    messages:
      wrong_content_type: "is the wrong content type"

file_mime_type_validator.rb

# Based on: https://gist.github.com/1009861

class FileMimeTypeValidator < ActiveModel::EachValidator
  MESSAGES  = { :content_type => :wrong_content_type }.freeze
  CHECKS    = [ :content_type ].freeze

  DEFAULT_TOKENIZER = lambda { |value| value.split(//) }
  RESERVED_OPTIONS  = [:content_type, :tokenizer]

  def initialize(options)
    super

  end

  def check_validity!
    keys = CHECKS & options.keys

    if keys.empty?
      raise ArgumentError, 'Patterns unspecified. Specify the :content_type option.'
    end

    keys.each do |key|
      value = options[key]

      unless valid_content_type_option?(value)
        raise ArgumentError, ":#{key} must be a String or a Regexp or an Array"
      end

      if key.is_a?(Array) && key == :content_type
        options[key].each do |val|
          raise ArgumentError, "#{val} must be a String or a Regexp" unless val.is_a?(String) || val.is_a?(Regexp)
        end
      end
    end
  end

  def validate_each(record, attribute, value)
    raise(ArgumentError, "A CarrierWave::Uploader::Base object was expected") unless value.kind_of? CarrierWave::Uploader::Base
    value = (options[:tokenizer] || DEFAULT_TOKENIZER).call(value) if value.kind_of?(String)
    return if value.length == 0

    CHECKS.each do |key|
      next unless check_value = options[key]

      if key == :content_type
        if check_value.is_a?(String) || check_value.is_a?(Regexp)
          do_validation(value, check_value, key, record, attribute)
        else
          check_value.each do |pattern|
            do_validation(value, pattern, key, record, attribute)
          end
        end
      end
    end
  end

  def help
    Helper.instance
  end

  class Helper
    include Singleton
    include ActionView::Helpers::NumberHelper
  end

  private

  def valid_content_type_option?(content_type)
    return true if %w{Array String Regexp}.include?(content_type.class.to_s)
    false
  end

  def do_validation(value, pattern, key, record, attribute)
    return if value.file.content_type.send((pattern.is_a?(String) ? "==" : "=~" ), pattern)

    errors_options = options.except(*RESERVED_OPTIONS)

    default_message = options[MESSAGES[key]]
    errors_options[:message] ||= default_message if default_message

    record.errors.add(attribute, MESSAGES[key], errors_options)

  end
end

post.rb

class Post < ActiveRecord::Base
  mount_uploader :image, ImagePostUploader

  validates :image,
    :file_mime_type => {
      :content_type => /image/
    }
end

Validation logic working fine with me, but the problem is that; active record every time run this validator even when no file is uploaded. So this obviously fails. Is there any way to tackle this situation?

Thanks for this! It works almost as I hoped. However, whenever I upload an unvalid file it raises ActiveRecord::RecordInvalid exception, instead of the usual flash notice.

Validation failed: Image Failed to manipulate with MiniMagick, maybe it is not an image? Original Error: Command ("identify -ping /var/folders/90/bt4jhsys70d71dfhr06q3crh0000gn/T/mini_magick20130722-17393-19zaaq0.html") failed: {:status_code=>1, :output=>"identify: delegate failed \"html2ps\" -U -o \"%o\" \"%i\"' @ error/delegate.c/InvokeDelegate/1065.\nidentify: unable to open image /var/tmp/magick-17517Qh--EFN7KBBh': No such file or directory @ error/blob.c/OpenBlob/2638.\nidentify: unable to open file /var/tmp/magick-17517Qh--EFN7KBBh': No such file or directory @ error/constitute.c/ReadImage/589.\n"}, Image is the wrong content type

Is it possible to correct this behavior? (I'm a rails newbie and need some points on how to do it myself)

Hi Denis, cool validator thanks for that.

But I use it like this:

  validates :icon,
    :file_mime_type => {
      :content_type => ['image/jpeg', 'image/png', 'image/gif'],
      :if => :has_icon?
    }

and in this case I've got 2 errors for each of invalid content_type.

that is why I've got update for you:

• def validate_each(record, attribute, value) looks like this for me:

  def validate_each(record, attribute, value)
    raise(ArgumentError, "A CarrierWave::Uploader::Base object was expected") unless value.kind_of? CarrierWave::Uploader::Base
    value = (options[:tokenizer] || DEFAULT_TOKENIZER).call(value) if value.kind_of?(String)
    return if value.length == 0

    CHECKS.each do |key|
      next unless check_value = options[key]
      do_validation(value, check_value, key, record, attribute) if key == :content_type
    end
  end

• and def do_validation(value, pattern, key, record, attribute)

  def do_validation(value, pattern, key, record, attribute)
    if pattern.is_a?(String) || pattern.is_a?(Regexp)
      return if value.file.content_type.send((pattern.is_a?(String) ? "==" : "=~" ), pattern)
    else
      valid_list = pattern.map do |p|
        value.file.content_type.send((p.is_a?(String) ? "==" : "=~" ), p)
      end
      return if valid_list.include?(true)
    end

    errors_options = options.except(*RESERVED_OPTIONS)

    default_message = options[MESSAGES[key]]
    errors_options[:message] ||= default_message if default_message

    record.errors.add(attribute, MESSAGES[key], errors_options)
  end