Carrierwave - Can't upload multiple images

I am using jQuery File Upload

Together with Carrierwave for uploading images, with latest stable
version of Ruby and Rails installed.

I managed to make it working by following this manual

And it works perfectly well for uploading single images, but
once I add HTML5 multiple attribute to allow uploading multiple images
at once and select multiple images, I get following error on each

NoMethodError in Advertiser::MediaController#create
undefined method `[]' for nil:NilClass

Every request seems absolutely same to me and I can't seem to figure out
why it can't upload multiple images but can upload single images, please
note that if I have multiple attribute enabled, I can't even upload a
single image but once I take it off, uploading works once again. jQuery
File Upload is perfectly capable of uploading multiple images and you
can check it out in their demo.

Any hints on where to dig would be very much appreciated.

Thank you in advance.

The error message should tell you the line number so you can see where
the problem is, you are calling [] on something that is nil. Have a
look at the Rails Guide on Debugging to get ideas on how to debug the