Save image to dropbox with data from canvas

I'm trying to write canvas data as an image (png) to my dropbox. I manage to get the data from canvas and to save a file to dropbox, but the file is not an image file it seams.

According to the documentation the image data should be converted to a arrayBuffer. That I'm doing using a function found here on Stackoverflow but something doesn't seem to work. Does anyone know what I'm doing wrong?

function _str2ab(str) {
    var buf = new ArrayBuffer(str.length*2); // 2 bytes for each char
    var bufView = new Uint16Array(buf);
    for (var i=0, strLen=str.length; i<strLen; i++) {
        bufView[i] = str.charCodeAt(i);
    }
    return buf;
}

function _savePicture () {

    //Get data from canvas
    var imageSringData = canvas.toDataURL('image/png');
    //Convert it to an arraybuffer
    var imageData = _str2ab(imageSringData);

    client.writeFile('/Public/the_image.png', imageData, function(error, stat) {
    if (error) {
        console.log('Error: ' + error);
    } else {
        console.log('File written successfully!');
    }
});

Here's some dropbox documentation. https://github.com/dropbox/dropbox-js/blob/stable/guides/snippets.md

Got it to work finally! I used another function to convert base64 to bufferArray and remove the 'data:image/png;base64,' from the string and it worked. Hopefully this will help someone else in the future.

function _base64ToArrayBuffer(base64) {
    base64 = base64.split('data:image/png;base64,').join('');
    var binary_string =  window.atob(base64),
        len = binary_string.length,
        bytes = new Uint8Array( len ),
        i;

    for (i = 0; i < len; i++)        {
        bytes[i] = binary_string.charCodeAt(i);
    }
    return bytes.buffer;
}

function _savePicture () {
    //Get data from canvas
    var imageSringData = canvas.toDataURL('image/png');
    //Convert it to an arraybuffer
    var imageData = _base64ToArrayBuffer(imageSringData);

    client.writeFile('/Public/the_image.png', imageData, function(error, stat) {
    if (error) {
        console.log('Error: ' + error);
    } else {
        console.log('File written successfully!');
    }
});