API to change Aseprite's supported file open extensions

[ZeWaka]

In a plugin I maintain, we support a non-standard file extension which is then loaded (by intercepting the file open command), parsed, and properly sent to aseprite as a PNG.
Unfortunately, in the open file dialog, our format is not in the list Aseprite passes to the OS:

Therefore, I'm seeking a way to modify or even just add to this list.
Maybe in app.fs, or even just app.

Just this right here: https://github.com/aseprite/aseprite/blob/main/src/app/file/file.cpp#L189-L197

[dacap]

Hi @ZeWaka, as part of aseprite/aseprite#1949 we'd like to add some API to register custom file formats. Some possible API was proposed in https://community.aseprite.org/t/registering-new-file-formats-in-extensions/16611