Manually uploading files via AJAX

To upload a file via AJAX (e.g. from an <input type='file'>) you need to wrap your params in a FormData Show archive.org snapshot object.

You can initialize a FormData using the contents of a form:

var form = document.querySelector('form.my-form') // Find the <form> element
var formData = new FormData(form); // Wrap form contents

Or you can construct it manually, param by param:

var fileInput = document.querySelector('form input[type=file]');
var attachment = fileInput.files[0];

var formData = new FormData(); 
formData.append('email', 'foo@bar.com');
formData.append('attachment', attachment, 'filename.jpg');

You can now send your FormData via AJAX like this:

var request = new XMLHttpRequest();
request.open('POST', '/my/url', true);
request.onload = function() { console.log("Server responded with %o", request.responseText) };
request.send(formData);

In jQuery you would send it like this:

$.ajax({
  url: '/my/url',
  type: 'POST',
  data: formData,
  contentType: false,
  processData: false
});

Note the contentType and processData options, which are needed so that jQuery does not try to serialize the FormData object.

Supporting legacy browsers

Short story:

• The solution above works in IE 10
• I recommend to offer a standard, non-AJAX form for older IEs
• All other solutions will bring you pain and long nights

Long story:

• FormData is available in Internet Explorer 10+ Show archive.org snapshot .
• While there are various Show archive.org snapshot polyfills Show archive.org snapshot for FormData, they're kind of pointless for our purpose. To be able to serialize files, they all require the File API Show archive.org snapshot , which again is available in Internet Explorer 10+ only.
• There are solutions like jQuery file upload Show archive.org snapshot that promise to work in legacy browsers. They mostly work by making a vanilla form submission to a hidden <iframe> and try to wrap a nice Javascript API around this. I found it really hard to make this work reliably.

Also see Dynamically uploading files to Rails with jQuery File Upload.