Well, let's try everything then.
Have you got the latest iPhone? Maybe they have enabled it in the latest version?
Try displaying a simple HTML like this:
<form>
<input type="file" name="abc">
</form>
and see if the Upload button is enabled on the latest one.
Also, try to Google for this issue - displaying Upload button for the HTML file fields - maybe you'll find something that we could not find.
Also, try to Google if it's possible to upload a file using Javascript. Again maybe you'll find something that we haven't.