Read a File using a FileReader returning a jQuery promise

gistfile1.js

function readFile(file) {
    var reader = new FileReader();
    var deferred = $.Deferred();

    reader.onload = function(event) {
        deferred.resolve(event.target.result);
    };

    reader.onerror = function() {
        deferred.reject(this);
    };

    if (/^image/.test(file.type)) {
        reader.readAsDataURL(file);
    } else {
        reader.readAsText(file);
    }

    return deferred.promise();
}

Use w Promise.all like any other:

//Assuming files is object of type FileList
var promises = [];
for (var i = 0; i < files.length; i++) {
    var promise = readFile(files[i]);
    promises.push(promise);
}
Promise.all(promises).then(function(arrayOfDataUrlsAndTextStrings) {
    //Do stuff
})

Exactly what I needed, thank you @jollytoad !