You can use this

http://www.brettle.com/neatupload

or an approach similar to the one in the component above

I can't help with the MAC thing, but we use BITS to upload files. It goes
over HTTP ports, can be throttled by the user, and it can restart uploads
that break so that whole upload doesn't start over.

http://msdn.microsoft.com/library/default.asp?url=/library/en-us/bits/bits/bits_
start_page.asp

Although it says it's for C and C++ developers there is a wrapper for C#
folks. It's my general experience that building two ways of doing this kind
of thing (one for Windows, another for MAC) might be better than one
complicated one that does both.