generic file and image models for uploaded files (See related posts)

class DbFile < ActiveRecord::Base
  IMAGE_TYPES = ['image/jpeg', 'image/pjpeg', 'image/gif', 'image/png', 'image/x-png']
  before_validation     :sanitize_filename
  validates_presence_of :size, :filename, :content_type

  class << self
    def new_file(file_data)
      content_type = file_data.content_type.strip
      (IMAGE_TYPES.include?(content_type) ? DbImage : DbFile).new \
        :data         => file_data.read,
        :filename     => file_data.original_filename,
        :size         => file_data.size,
        :content_type => content_type
    end
  end

  protected
  def sanitize_filename
      # NOTE: File.basename doesn't work right with Windows paths on Unix
      # get only the filename, not the whole path
      filename.gsub! /^.*(\\|\/)/, ''

      # Finally, replace all non alphanumeric, underscore or periods with underscore
      filename.gsub! /[^\w\.\-]/, '_'
  end
end

require 'rmagick'
require 'base64'
class DbImage < DbFile
  def data=(file_data)
    with_image(file_data, true) do |img|
      self.width  = img.columns
      self.height = img.rows
    end
  end

  def with_image(file_data = nil, save_image = false, &block)
    img = Magick::Image::read_inline(Base64.b64encode(file_data || self.data)).first
    block.call(img)
    write_attribute('data', img.to_blob) if save_image
    img = nil
    GC.start
  end
end

# returns DbImage if content_type matches
db_file = DbFile.new_file(params[:file][:data])
db_file.save

# raw binary image data
File.open('my_file', 'w') { |f| f.write(db_file.data) }

# Image resizing with rmagick
# automatically creates RMagick::Image and 
# invokes GC.start
db_file.with_image do |img|
  img.scale(.25)
  img.write('thumb.jpg')
end