You can use the Wscript Shell object to activate the dialog that is opened when a file upload is initiated. The process for bringing a window into focus looks like this. You'll need to adjust it accordingly to fit your code. Primarily, you need to supply the window title. I'm not on Windows right now to test, but it would typically be something like "Choose File to Upload"
Set objExplorer = CreateObject("InternetExplorer.Application")
objExplorer.Visible = True
Set objShell = CreateObject("WScript.Shell")
objShell.AppActivate objExplorer.Name
Set objShell = Nothing
Set objExplorer = Nothing
You might also try using SendKeys without activating the window first. The browse dialog should be application modal. As long as your webpage tab is activated, it should have focus anyway. Don't forget that buttons can be pressed via their keyboard shorcuts as well.