module AssetsHelper

  def render_asset(asset, custom_class=nil)
    case(asset.mime.split('/').first)
    when 'image'
      content_tag(:img, nil, src: asset.url, alt: asset.alt, title: asset.title, class: ['media-object', custom_class].join(' '))
    else
      link_to asset.url, asset.url
    end
  end

end

module AssetsHelper

  def render_asset(asset,custom_class)
    case(asset.mime.split('/').first)
    when 'image'
      content_tag(:img, nil, src: asset.url, alt: asset.alt, title: asset.title, class: 'media-object '+custom_class)
    else
      link_to asset.url, asset.url
    end
  end

end

--- 
+++ 
@@ -1,9 +1,9 @@
 module AssetsHelper
 
-  def render_asset(asset,custom_class)
+  def render_asset(asset, custom_class=nil)
     case(asset.mime.split('/').first)
     when 'image'
-      content_tag(:img, nil, src: asset.url, alt: asset.alt, title: asset.title, class: 'media-object '+custom_class)
+      content_tag(:img, nil, src: asset.url, alt: asset.alt, title: asset.title, class: ['media-object', custom_class].join(' '))
     else
       link_to asset.url, asset.url
     end
Make custom_class optional in render_asset helper
