He had explained, it is SVG. You simply remove these masks from the file or change transparency.

I've prompted ChatGPT to make a bit more detailed explanation: https://chat.openai.com/share/42e55091-18c2-421e-9452-930114...

You can probably prompt it to further to generate python code and unmask the file for you, in the interpreter.

Incidentally, this use of GPT4 is somewhat similar to the threat model that they are studying. I'm a bit surprised that they've used plain GPT-4 for the study, rather than GPT-4 augmented with tools and a large dataset of relevant publications.