attachment_fu is a plugin by Rick Olson (aka technoweenie <http://techno-weenie.net>) and is the successor to acts_as_attachment.

attachment_fu makes file uploads a snap.  To get a basic run-through of its capabilities, check out Mike Clark's tutorial <http://clarkware.com/cgi/blosxom/2007/02/24#FileUploadFu>.  This document will focus on the details of the model portion of attachment_fu.

  There are three storage options for files uploaded through attachment_fu:
    file system   # :file_system
    database file # :db_file
    Amazon s3     # :s3
  
  For all three of these storage options a table of metadata is required.  This table will contain information about the file (hence the 'meta') and its location.  This table has no restrictions on naming, unlike the extra table required for database storage, which must have a table name of db_files (and by convention a model of DbFile).
  
  Fields for attachment_fu metadata tables...
    in general:
      size,         :integer  # file size in bytes
      content_type, :string   # mime type, ex: application/mp3
      filename,     :string   # sanitized filename
    that reference images:
      height,       :integer  # in pixels
      width,        :integer  # in pixels
    that reference images that will be thumbnailed:
      parent_id,    :integer  # id of parent image (on the same table, a self-referencing foreign-key).
                              # Only populated if the current object is a thumbnail.
      thumbnail,    :string   # the 'type' of thumbnail this attachment record describes.  
                              # Only populated if the current object is a thumbnail.
                              # Usage:
                              # [ In Model 'Avatar' ]
                              #   has_attachment :content_type => :image, 
                              #                  :storage => :file_system, 
                              #                  :max_size => 500.kilobytes,
                              #                  :resize_to => '320x200>',
                              #                  :thumbnails => { :small => '10x10>',
                              #                                   :thumb => '100x100>' }
                              # [ Elsewhere ]
                              # @user.avatar.thumbnails.first.thumbnail #=> 'small'
    that reference files stored in the database (:db_file):
      db_file_id,   :integer  # id of the file in the database (foreign key)
      
  Field for attachment_fu db_files table:
    data, :binary # binary file data, for use in database file storage