Ink file picker callback called too early. How to detect when the file is available?

Ink file picker callback called too early. How to detect when the file is
available?

When uploading a file using filepicker.io, the filepicker.pick success
callback is getting called before the file is actually available. Here's
the code:
filepicker.pick({
mimetypes: ['image/*'],
container: 'modal',
services:['COMPUTER', 'FACEBOOK', 'INSTAGRAM', 'WEBCAM']
},
function(inkBlob){
$('img.foo').attr('src', inkBlob.url);
},
function(FPError){
console.log(FPError.toString());
});
I get a url in the inkBlob that comes in the callback, but sometimes if I
insert that url into the dom (as above), I get a 404. Other times it
works. I'm looking for a reliable way to know when I can use the file
returned by filepicker. I figured the success callback was it, but there
seems to be this race condition.
I realize I could wrap the success callback in a setTimeout, but that
seems messy, and I'd like to not keep the user waiting if the file is
actually available.