Fix bug with filename not being shown w/ mongoid and carrierwave

`Ckeditor::AttachmentFile#filename` returns nil, but 
`Ckeditor::AttachmentFile.first.data_file_name` returns the proper
filename (ie : the filename gets saved). Changing the geneator to this
ensure the name was never nil, both when uploading a new file and when
loading the `index` action.