Feature: Support for file types other than PNG

[cosimo-rip]

The default behavior for Candy Machine is to take PNG assets. Treat Toolbox was built to match this.
However, it has quickly become a common request to be able to use Treat Toolbox to manage collections of other types (animated GIFs, videos, etc).

First task is to ensure that exported content can be handled by Candy Machine in these formats.

We'll also need to see what assumptions in Treat Toolbox need to be adjusted. These file types don't typically come with expectations of generative art. Usually, an artist has provided 500 finished GIFs, or 500 finished videos.