Writedown for iPad and iPhone allows creating and opening files through the x-callback-url specification.

x-callback-url The URL scheme is compliant to the x-callback-url specification. Visit the x-callback-url website
Application homepage: http://serpensoft.com
iTunes link: https://itunes.apple.com/app/id512169690
HTML Link example:
Native application URL string example: writedown://x-callback-url/new?name=foo.txt&text=bar
Extended example:
Tags: write, writedown, markdown