title: How to intercept and add parameters to POST using javascript and ajax?
body: <p>I have a pretty standard upload form - user picks a file, then hits the upload button. But now I need to add data to the post - after the user picks the file, but obviously before the post leaves the browser. Note that the post is a direct post to a third party (Amazon S3).</p>
  
  <p>Is there a way to make a form with a file picker (or something that looks like one), then the user picks a local file, then hits a button, => javascript calls my server, gets a response, builds the 'real' post and then sends this new post to a third party server. </p>
  
  <p>Basically, until the user picks the file, I don't know a few things (mime type). I know that browsers send this info, but Amazon AWS pre signed posts ignore what the browser says. </p>
  
  <p>I can hit my server (ruby sinatra) with an ajax call from the javascript, which will return some JSON, etc to the script, which will then post to Amazon S3.   </p>
  
  <p>My problem could just be my newbieness to javascript...</p>