I’m sure it also depends on the OS, whether you have a “theme” installed, which one, etc.
Opera is apparently able to style them, since they look (vaguely) like the rest of the form buttons on PVB (except for the font).
If it was as you say, it should be a standard “Windows Classic” button for me. I use XP with no theme (i.e. Win9x/2K style) and I also have Opera set to use no “skin”. Also, other pages have standard “beveled” buttons on their upload forms. Here’s one from VirusTotal: