NoMethodError in CollectionsController#create undefined method `file' for nil:NilClass

Question

I'm trying to upload a pdf file to dropbox through rails and convert the pages in the pdf into jpeg images and store those images in the same dropbox folder. I tried using Docsplit gem but i dont know how to address the file inthe drop box in code.

Docsplit.extract_images(@collection.file.url, :format => [:jpeg])

file.url shoul return the file right? or any other ways??

thanks,