Adds asset management capabilities to your model.

=== Usage example:

class Page
  asset_field :asset,
              :directory  => "#{RAILS_ROOT}/public/assets/images",
              :extensions => %w( .swf .qt ),
              :filename   => :permalink
end

• [R,W] asset_tempfile

• Page#asset_file_on_disk Full pathname of the file on disk.
• Page#asset_exist? Does the file actually exist on disk?
• Page#write_asset_file Write the file to the path generated from the asset field’s directory option and the method referenced by the filename option.
• Page#destroy_asset_file Destroy the asset file.

• :directory Directory to save this field’s assets in. Defaults to ”#{RAILS_ROOT}/public/assets”.
• :filename A symbol referencing the name of a method returning a string returning a filename (minus the file’s extension). It is of note that this could simply be a reference to another attribute.
• :extensions An array of allowed file extensions. Omitting this option allows all file extensions.