stack level too deep when mount on a collumn named "file"

If you have you uploader mounted on a column named ```file```, like this:

```ruby
class Attachment < ActiveRecord::Base
  attr_accessible :name, :file, :size, :attachable_id, :attachable_type, :content_type
  mount_uploader :file, AttachmentUploader
  ... 
end
```

You will see a ```SystemStackError (stack level too deep)``` when trying to use ```create```, like this:

```
Attachment.new(params[:attachment])
```

It's the same problem related in #10 and #18. I don't know why they closed the other issues, when it's clearly a problem in the gem(in my opinion), since it works with regular carrierwave(and it's pretty hard for a user to figure it out).