PXO format is missing from supported files in the open dialog in the web version

[novhack]

Pixelorama version:
Itch.io web version

Issue description:
When opening a file with the web version of Pixelorama the default filter shows only supported files and .pxo files are hidden.
Adding pxo extension to supported formats should solve the issue.

It would be best to add all supported formats to this line:

HTML5FileExchange.gd:31:

input.setAttribute("accept", "image/png, image/jpeg, image/webp");