Add support for image/file fields in templates

Problem/Motivation

Assets and logs both have image/file fields - we should support them!

Steps to reproduce

N/A

Proposed resolution

I'm hoping this will be a simple add, using existing file upload form elements. Need to think if there are any unique settings or otherwise that we should include.

Remaining tasks

Implement

User interface changes

None

API changes

None

Data model changes

None