Get The File Size Before Uploading Perhaps Using AJAX

I have figured out that javascript cannot obtain the filesize of a to-be-uploaded file because it is a security risk and it is not allowed access to the clients HD. But I want to see if it is possible to get the file size before uploading the file, perhaps using AJAX.

So by looking at the requests, when you click the submit button on a form that contains a input type=file, the Request actually has the filesize inside of it, so you could therefore get the first request, cancel the request there, and send the file-size data back to the client without actually uploading the file.

I want to do this via AJAX so that it doesn't post back and also without clicking the submit button. It seems impossible to create a submit request with javascript because there is no way to get the file size. But is there a way to fake a submit? How does the submit button get the information? Is there any documentation on how the submit button actually works?