"Cannot open the file because the file format or extension is invalid" opening Office files

 We have identified the malware as a new family of ransomware

• Infection: Trojan:Win32/Bumat!rts AVSubmit 
  Detectable with AM definitions version:  1.157.1559.0 or later
  http://www.microsoft.com/security/portal/threat/encyclopedia/entry.aspx?Name=Trojan%3aWin32%2fBumat%21rts&ThreatID=-2147341227

• Infection: Trojan:Win32/Crilock.A. 
  Detectable with AM definitions version:  1.157.1542.0 or later
  http://www.microsoft.com/security/portal/threat/encyclopedia/Entry.aspx?Name=Trojan%3aWin32%2fCrilock.A

Definition Information:

    Latest public Endpoint AM defs:  1.157.1542.0 (Expect several hours later for 1.157.1559.0 or higher)
      http://www.microsoft.com/security/portal/definitions/whatsnew.aspx

    Latest Prerelease AM defs: 1.157.1542.0 (Expect several hours later for 1.157.1559.0 or higher)
      http://www.microsoft.com/security/portal/shared/prereleasesignatures.aspx

The signatures are now updated. You should be able to use Safety Scanner to detect and clean it. It does NOT clean your documents, so you will want to restore from a backup after cleaning the malware from your system.

• Microsoft Safety Scanner http://www.microsoft.com/security/scanner/en-us/default.aspx

Upon further analysis of the files that have been submitted to us for investigation, the analysts

have determined that the files are encrypted with a private and public key.

Unless the private key is available, the files will not be able to be recovered.

The private key is more than likely held by the attacker.The premise of ransom ware such as this is

that if a person pays the ransom, a key is provided to unlock the files.

The best course of action is to clean up the malware and then restore files from a backup.