Resize Image File with Javascript and HTML5 in Browser

In my Webapp i let the User choose a picture from their harddrive through an input form. Then i want to resize it. the html code looks like this:

<form style="margin-top: 15px;">
<input type="file" name="pic" id="product_pictureUploadModal" accept="image/jpeg, image/png, image/jpg" />
</form>

I get the file attached to that input as follows:

var fileUploadControl = $("#product_pictureUploadModal")[0];
var file = fileUploadControl.files[0];

Now i want to resize the image the user chose not changing its aspect ratio.

I´m doing that using a canvas right now: Using following code:

     function resizeImage(file){

        var img = document.createElement("img");
        img.src = window.URL.createObjectURL(file);
        var canvas = document.createElement('canvas');

        var MAX_WIDTH = 250;
        var MAX_HEIGHT = 1500;
        var width = img.width;
        var height = img.height;

        if (width > height) {
            if (width > MAX_WIDTH) {
                height *= MAX_WIDTH / width;
                width = MAX_WIDTH;
            }
        } else {
            if (height > MAX_HEIGHT) {
                width *= MAX_HEIGHT / height;
                height = MAX_HEIGHT;
            }
        }
        canvas.width = width;
        canvas.height = height;
        var ctx = canvas.getContext("2d");
        ctx.drawImage(img, 0, 0, width, height);

        var fileRezized = canvas.toDataURL('image/jpeg');
        return fileRezized; 
    }

The problem i have now is, that my backend, named Parse, does not accept String as an file upload parameter. Before resizing the image i can upload it just fine using a Parse.File. But afterwards i get the following errormessage: uncaught exception: Creating a Parse.File from a String is not yet supported.

I hope you guys can help me. I believe what I´m trying to do shouldnt be that hard. Am i even on the right track? Where should read into, what should i start to learn to be able to do this?

greetings from germany, marvin.