Save downloaded file to iCloud Drive with Safari on iOS8
For anyone else who comes acros this question via Google or some other search engine, there is a way!  

The most excellent app, Workflow, once again comes to the rescue.  You can create a file downloading workflow that works perfectly:  

http://www.guidingtech.com/36337/file-downloader-safari-ios-8/  

I'm on iOS 9 and works great.  I really need to pay attention to Workflow more - it really is the glue that solves many of the little niggling edge cases for iOS lacking what is easily done on a desktop.