I could be wrong, but I don't think security settings would allow you to arbitrarily pick a file from a user's machine and upload it to your server.
Since you can hide forms and auto-submit data without user interaction, imagine going to a website that tries to pick a file from your machine and download it...
I am pretty sure that the user has to select the file, thereby "attaching" it to the form in order for an upload to work.
You might be able to do it with some sort of ActiveX control, as you need the user's permission to install that, but any other way is not doable (me thinks)
TIP: trying googling the answer before posting, you'll find that more times than not someone else somewhere has had the same request and posted an answer online.
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I have recently been semi-converted to ensuring all my code (well most of it) works in both javascript and non-javascript enabled browsers