File uploading via FileAPI

Just to be perfectly clear, FileReader isn't downloading anything. It is asynchronously reading the File contents (as it appears on disk) into memory.

To do what you want, just keep track of the read files and call a callback when they're all done:

document.querySelector('[type="file"]').change = function(e) {
  handleFiles(toArray(e.target.files), function(results) {
    // results is an Array containing the FileReader results.
    alert('Done!');
  });

function toArray(list) {
  return Array.prototype.slice.call(list || [], 0);
}

function handleFiles(files, callback) {
  var results = [];

  files.forEach(function(file, i) {
    var reader = new FileReader();

    // Closure to capture the file information.
    reader.onload = function(e) {
      results.push(e.target.result);
      if (results.length == files.length) {
        callback(results);
      }
    };
    reader.readAsDataURL(file);
  });
}

Try it: http://jsbin.com/epatel/3/edit#html,live