FSDT hosts their files on Dropbox, a cloud storage provider. The downloads will therefore be from the Dropbox servers. The warning is part of Microsoft SmartScreen (a feature in Internet Explorer 9, Windows Live Messenger, and Windows 8), which you can safely ignore for downloads you know to be safe (in this case, the FSDT installers). I believe you're confusing the source of the download and the digital signature, as the latter is signed as "VIRTUALI SAS DI COLAPICCHIONI UMBERTO & C".