Nope. You don't need to do anything special.

The fileupload works the same way wether Windows, Mac or Linux.

In theory if the user is using a W3C complient browser (regardless of
the client OS) then all ASP.NET generated pages should be accessible.

In practice there may be anomalies with some browsers particularly
older ones and if you want to try and accommodate them VS does have
verification tools which will flag up any HTML that might cause a
problem. However this is only applicable to manually entered markup in
the .aspx file. All web server controls are rendered by ASP.NET in
such a way that most reasonably up to date browsers should cope with.
The only time that the developer has control of this is with custom
web controls.

HTH