Creating a Blob from a Windows IBuffer or RandomAccesStream

gistfile1.js

// If you are starting with a file from a picker, something like this should work:

var picker = new Windows.Storage.Pickers.FileOpenPicker();
picker.fileTypeFilter.replaceAll(['.jpg', '.jpeg']);

picker.pickSingleFileAsync().then(function (file) {
    return file.openAsync(Windows.Storage.FileAccessMode.read);
}).done(function (ras) {
    var blob = MSApp.createBlobFromRandomAccessStream('image/jpeg', ras);

    var img = document.createElement('img');
    document.body.appendChild(img);
    img.src = URL.createObjectURL(blob);
})

// If you are starting with a WinRT buffer, the below works, but there may be something more direct as well:

var picker = new Windows.Storage.Pickers.FileOpenPicker();
picker.fileTypeFilter.replaceAll(['.jpg', '.jpeg']);

picker.pickSingleFileAsync().then(function (file) {
    return Windows.Storage.FileIO.readBufferAsync(file);
}).done(function (buffer) {
    var reader = Windows.Storage.Streams.DataReader.fromBuffer(buffer);
    var bytes = new Uint8Array(buffer.length);
    reader.readBytes(bytes);
    var blob = new Blob([bytes], { type: 'image/jpeg' });

    var img = document.createElement('img');
    document.body.appendChild(img);
    img.src = URL.createObjectURL(blob);
})