If the webviewer is not an option because the thing you're trying to do must be done unattended then you could use plugins like the troi-url and troi-text or the 360works-scriptmaster. With these plugins installed you can have filemaker visit websites and/or ftp-servers to download or to upload data.
Another possible solution is to use cUrl (which is integrated in MacOSX 10.5.x and must be installed in windows), you use actually the commandline of your OS to send a certain command to cUrl which will upload or download to or from a website.
You have a bit of reading to do though before you have this running. I have not found any ready to use sulotions only a lot of tips. To use the plugins you can download trails their websites, Troi: http://www.troi.com/, 360-works: http://www.360works.com/products/ and to use cUrl : http://curl.haxx.se/