Can't shed any light on this particular issue (the file upload thang), but regarding Opera in general... it has great XHTML / CSS support, but a really lousy handling of the Document Object Model (for this reason, any half-complex JScript / VBScript may give Opera some problems).

As i say; no idea why that would be putting the kibosh on file uploads (it usually just chokes when you do things like putting little JScript controls in the page to show/hide sections of text). But something to consider perhaps.