In the current version of Rebex File Transfer pack, resuming file transfer is very easy - once connected and authenticated, the following line of code is all you need:
client.Upload("source.zip", "/target_dir", TraversalMode.NonRecursive, ActionOnExistingFiles.ResumeIfPossible)
Unfortunately, this method was not available in v1.0.3127.0 (released in 2008). Instead, check out the ResumableTransfer sample app that came with the install package - the
BackgroundUpload methods (in MainForm.cs/MainForm.vb file) demonstrate how to resume a file transfer and describe the process in detail.