Your browser seems to store the pk3 as a zip file. Just rename map-thismap.zip to map-thismap.pk3 in that case.
Another way is to start the game, open console, type /set cl_allowdownload 1
and autodownload the file from the server. It is pretty slow though.
If you have DSL you should type /set rate 25000
in console for less lag and faster download.
After that just connect to the server and it will load the map.
Danny