Update URLField to handle file urls

[beta1042]

I work in a corporate sharepoint site and we have list content linked back to actual files on our network (more of just a reference list than a live edited list). Issue is that the hyperlinks are linked to href='file://...' links.

I ended up editing the lists/types.py file, URLField class to accept "file:" url links.
was:
...value['href'].startswith(prefix) for prefix in ('mailto:', 'http:', 'https:')
is:
...value['href'].startswith(prefix) for prefix in ('mailto:', 'http:', 'https:', 'file:')

Any reason why this couldn't be standard behavior moving forward?