CarrierWave and correct file extension depending on its contents

Question

How can I make CarrierWave add correct extension to filename depending on its contents? For example, if I upload file \"logo\" (PNG file without extension) CarrierWave shoul

Answer 1

The problem is in determining the correct content in the first place. Carrierwave uses the MimeType gem which determines its mime-type from the extension. Since, in your case the extension is incorrect you need an alternate way of getting the correct mime-type. This is the best solution I was able to come up with, but it depends on the ability to read the image file using the RMagick gem.

I ran into this same problem and had to override the default set_content_type method for my uploader. This assumes you have Rmagick gem in your Gemfile, so that you can get the correct mime-type from reading the image, as opposed to making a best guess.

Note: This is particularly useful if the image is being used by Prawn which supports only JPG and PNG images.

Uploader Class:

process :set_content_type

def set_content_type #Note we are overriding the default set_content_type_method for this uploader
  real_content_type = Magick::Image::read(file.path).first.mime_type
  if file.respond_to?(:content_type=)
    file.content_type = real_content_type
  else
    file.instance_variable_set(:@content_type, real_content_type)
  end
end

Image Model:

class Image < ActiveRecord::Base
  mount_uploader :image, ImageUploader

  validates_presence_of :image
  validate :validate_content_type_correctly

  before_validation :update_image_attributes

private
  def update_image_attributes
    if image.present? && image_changed?
      self.content_type = image.file.content_type
    end
  end

  def validate_content_type_correctly
    if not ['image/png', 'image/jpg'].include?(content_type)
      errors.add_to_base "Image format is not a valid JPEG or PNG."
      return false
    else
      return true
    end
  end
end

In your case you can add an additional method that changes the extension based on this correct mime-type (content_type).