Where is defined fetchFile?

I'm using version 12.15 and in my code here is what i'm doing:

In my _Layout.cshtml page i have add the scripts:

```

document.addEventListener('DOMContentLoaded', async () => {
if (typeof FFmpegWASM?.FFmpeg !== "function") {
console.log("FFmpeg not available");
return;
}

try {
const ffmpeg = new FFmpegWASM.FFmpeg();
await ffmpeg.load({
log: true,
corePath: '~/ffmpeg-core/umd/ffmpeg-core.min.js'
});
}
catch (e) {
console.log(e);
}
}

```

But i didnt find where fetchFile is defined.
Am I missing something?