Send custom data with dropzone.js on each File Upload

I got it. This is what I had to use

myDropzone.on('sending', function(file, xhr, formData){
    formData.append('userName', 'bob');
});

Abhinav has the right and working answer I only want to give a second option to use it in the options object (for example if you have multiple Dropzone sections on one page.)

myDropzone.options.dropzoneDivID = {
    sending: function(file, xhr, formData){
        formData.append('userName', 'Bob');
    }
};

In case you have a nested payload object - e.g. to add a name to your file and your api only accepts something like this

{
    someParameter: {
        image: <my-upload-file>,
        name: 'Bob'
    }
}

your dropzone setup would look like this

var myDropzone     = new Dropzone("div#attachment", { 
    url: uploadFilePath,
    paramName: 'someParameter[image]'
});

myDropzone.on('sending', function(file, xhr, formData){
    formData.append('someParameter[image]', file);
    formData.append('someParameter[userName]', 'bob');
});

I only added this as there was no example for nested parameters documented since now.