form.js

$(document).ready(function(){
    //this assumes you have a file field in your form with id=image
    //when this field is updated, run the function
    $('#image').change(function(e){
        var file = this.files[0];
        var form = new FormData();
        form.append('files', file);
        $.ajax({
            url : "https://api.knack.com/v1/applications/[YOUR APP ID]/assets/image/upload",
    headers: {"x-knack-application-id":"YOUR APP ID",
        "x-knack-rest-api-key": "YOUR API KEY"},
            type: "POST",
            cache: false,
            contentType: false,
            processData: false,
            data : form,
            success: function(response){
                $('.result').html(response.html)
            }
        });
    });
});