Antivirus complains / deletes the file

This is a false positive. FlowSync modifies the registry and services — behaviour that resembles a virus, so Windows Defender, Kaspersky and Avast may block it.

The fix: add FlowSync.exe to your antivirus exclusions:

• Defender: Settings → Windows Security → Virus & threat protection → Manage settings → Add or remove exclusions.
• Kaspersky: Settings → Threats and exclusions → Manage exclusions.

The file checksum is shown after you click Download — you can verify it yourself.