It is almost certainly a typo-ridden, SEO-poisoned, or malware-laced string. Legitimate Fortinet firmware requires a support account and follows a clean naming convention. If you need FortiManager to "work," contact Fortinet sales or a certified reseller.
Hackers often name malware after enterprise security tools (like FortiManager). A file named fmgvm64...work.zip is likely a Trojan. fmgvm64kvmv6build1183fortinetoutkvmzip work