From Qt documentation:

bool QWebPage::extension ( Extension extension, const ExtensionOption * option
= 0, ExtensionReturn * output = 0 ) [virtual]

This virtual function can be reimplemented in a QWebPage subclass to provide
support for extensions. The option argument is provided as input to the
extension; the output results can be stored in output.

The behavior of this function is determined by extension. The option and output
values are typically casted to the corresponding types (for example,
ChooseMultipleFilesExtensionOption and ChooseMultipleFilesExtensionReturn for
ChooseMultipleFilesExtension).

You can call supportsExtension() to check if an extension is supported by the
page.