Rails migration for switching from file_column to paperclip.

migrate_users_to_paperclip.rb

require 'fileutils'

class MigrateUsersToPaperclip < ActiveRecord::Migration

  def up
    # Rename the old "mugshot" column to "old_file_name", since User.mugshot will now try to do Paperclip stuff
    rename_column :users, :mugshot, :old_file_name

    # Rename asset directories to pluralize
    File.rename("public/system/user","public/system/users")
    File.rename("public/system/users/mugshot","public/system/users/mugshots") 

    User.all.each do |u|
      unless u.old_file_name.blank? && u.old_file_name != '/mugshots/original/missing.png'
        # If an image was set by file_column, copy the relevant parameters to the new Paperclip columns
        # IMPORTANT: Set config.active_record.whitelist_attributes = false in config/application.rb before running this script
        u.update_attributes(
          :mugshot_file_name => u.old_file_name,
          :mugshot_content_type => 'image/jpeg',
          :mugshot_updated_at => Time.now.utc,
          :mugshot_file_size => File.stat("public/system/users/mugshots/#{u.id}/#{u.old_file_name}").size
        )

        # Now rename the image file to add the original_ prefix
        File.rename("public/system/users/mugshots/#{u.id}/#{u.old_file_name}","public/system/users/mugshots/#{u.id}/original_#{u.old_file_name}")

        # Also need to rename the various files sizes. 
        # File column stores them like this:
        # public/system/users/mugshots/2/thumb/antun_jump.jpg
        # 
        # Paperclip stores them like this:
        # public/system/users/mugshots/2/thumb_antun_jump.jpg
        u.mugshot.styles.each do |s|
          style_name = s[0]
          FileUtils.mv("public/system/users/mugshots/#{u.id}/#{style_name}/#{u.old_file_name}", "public/system/users/mugshots/#{u.id}/#{style_name}_#{u.old_file_name}")
        end
      end
    end

    # And finally remove the old_file_name column, which we don't need any more
    remove_column :users, :old_file_name
  end

  def down
    # Rename the old "mugshot" column to "old_file_name", since User.mugshot will now try to do Paperclip stuff
    if column_exists? :users, :old_file_name 
    then
      # Previous migration exited with error, so just restore that temporary column.
      rename_column :users, :old_file_name, :mugshot
    elsif column_exists? :users, :mugshot
    then
      # Previous migration exited with error, so just restore that temporary column.
      # Do nothing.
    else
      # Previous migration was completed, so need to restore the column altogether.
      # However, we can't create a :mugshot column yet, because at this point, 
      # Paperclip uses :mugshot, and will complain if you try to update_attributes
      # on it.
      add_column :users, :mugshot_temp, :string
      User.all.each do |u|
        unless u.mugshot_file_name.blank?
          u.update_attributes(
            :mugshot_temp => u.mugshot_file_name,
            :mugshot_file_name => nil,
            :mugshot_content_type => nil,
            :mugshot_updated_at => nil,
            :mugshot_file_size => nil
          )
        end
      end
      rename_column :users, :mugshot_temp, :mugshot
    end

    # Rename asset directories to singularize
    if File.directory? "public/system/users/mugshots"
    then
      File.rename("public/system/users/mugshots","public/system/users/mugshot") 
    end
    if File.directory? "public/system/users"
    then
      File.rename("public/system/users","public/system/user")
    end

    # Use temporary name for column so that we don't trip-up over default paperclip functionality
    rename_column :users, :mugshot, :mugshot_temp

    # Rename size assets back to file_column style:
    # public/system/users/mugshots/2/thumb/antun_jump.jpg
    # 
    # Paperclip stores them like this:
    # public/system/users/mugshots/2/thumb_antun_jump.jpg
    User.all.each do |u|
      unless u.mugshot_temp.blank?

        File.rename("public/system/user/mugshot/#{u.id}/original_#{u.mugshot_temp}", "public/system/user/mugshot/#{u.id}/#{u.mugshot_temp}")

        u.mugshot.styles.each do |s|
          style_name = s[0]
          FileUtils.mkpath("public/system/user/mugshot/#{u.id}/#{style_name}")
          FileUtils.mv("public/system/user/mugshot/#{u.id}/#{style_name}_#{u.mugshot_temp}", "public/system/user/mugshot/#{u.id}/#{style_name}/#{u.mugshot_temp}")
        end
      end
    end

    rename_column :users, :mugshot_temp, :mugshot

  end
end

I think you mean || not && on line 14. Thanks for this!

What about change content_type to:

MIME::Types.type_for(u.old_file_name).first.content_type ?