Running script.
<script>
my_file = AppInventor.getWebViewString(); // INPUT
// import my_file from '/storage/emulated/0/Android/data/edu.mit.appinventor.aicompanion3/files/abc_ASD.txt';
//           my_file = 'file://storage/emulated/0/Android/data/edu.mit.appinventor.aicompanion3/files/abc_ASD.txt';

fetch(my_file).then(function (response) {
    // get the data as a Blob
 if (!response.ok) throw alert("fetch failed");
    return response.arrayBuffer();
})
.then(function (ab) {
    var data = new Uint8Array(ab);
   AppInventor.setWebViewString("" + data); // OUTPUT
});
</script>