Note:

The answer(s) below reflect the state of legacy browsers in 2009. Now you can actually set the value of the file input element dynamically/programatically using JavaScript in 2017.

See the answer in this question for details as well as a demo:
How to set file input value programatically (i.e.: when drag-dropping files)?

What I need to do is to programmatically send a POST request to a service from javascript code from an application that uses a .NET WebBrowser, which is basically an embedded Internet Explorer. This service requires one of the fields to be a file.

So either, is there any way to set the value of a file input in a form, given that I have the content of the file, such as in an array of bytes (and not the path of the file; I'm not trying to steal files from users here :) ).

Or perhaps a way to explicitly create and send the POST request instead of creating a FORM dynamically?

I need to use JavaScript code that will be run in my WebBrowser (think IE). Is this possible?