使用Paperclip自定义缩略图文件类型

I'm using Paperclip with a Ruby on Rails to attach assets to a model, these assets can be any file type and currently thumbnails are only being generated if the asset is an image. I'd like to be able to display a different default image for other files, either by generating a thumbnail of the files on upload, or setting something up with the default_url but so far I can't find any resources to help with this and am getting no where on my own.I'm using Paperclip with a Ruby on Rails to att