Yes it is supported.
From the IIS 5 Help file:
/iishelp/iis/htm/core/iivsovr.htm#ftpresume
FTP Restart
FTP restart addresses the problem of losing a network connection while downloading files. Clients that support FTP restart need only re-establish their FTP connection using the REST command and the file transfer will automatically pick up where it left off.
Note The IIS 5.0 implementation of FTP restart is not enabled when using FTP to download wildcard requests (MGET), uploading files to a server (PUT), or downloading files larger than 4 gigabytes.
