Force file extension to match file type: registry value extensionhardeningUserHKCU\software\policies\microsoft\office\16.0\excel\security

Policy ID: L_Forcefileextenstionstomatch
Scope: User Configuration
Supported on: At least Windows Server 2008 R2 or Windows 7
Source: Microsoft 365 Apps / Office Administrative Templates (version 5568.1000)
Category: Microsoft Excel 2016 › Excel Options › Security

Registry

extensionhardening

REG_DWORDUser

HKCU\software\policies\microsoft\office\16.0\excel\security Current key

Enabled value: Depends on the option value
Element: -

Options
Item	Stored value
Allow different	0
Allow different, but warn	1
Always match file type	2

Apply with .reg · CMD · PowerShell

.reg

Windows Registry Editor Version 5.00

[HKEY_CURRENT_USER\software\policies\microsoft\office\16.0\excel\security]

; Options (use only one)
; Allow different
"extensionhardening"=dword:00000000
; Allow different, but warn
"extensionhardening"=dword:00000001
; Always match file type
"extensionhardening"=dword:00000002

CMD

:: Options (use only one)

:: Allow different
reg add "HKCU\software\policies\microsoft\office\16.0\excel\security" /v "extensionhardening" /t REG_DWORD /d 0 /f
:: Allow different, but warn
reg add "HKCU\software\policies\microsoft\office\16.0\excel\security" /v "extensionhardening" /t REG_DWORD /d 1 /f
:: Always match file type
reg add "HKCU\software\policies\microsoft\office\16.0\excel\security" /v "extensionhardening" /t REG_DWORD /d 2 /f

PowerShell

# Common: create the key
New-Item -Path 'HKCU:\software\policies\microsoft\office\16.0\excel\security' -Force | Out-Null

# Options (use only one)
# Allow different
New-ItemProperty -Path 'HKCU:\software\policies\microsoft\office\16.0\excel\security' -Name 'extensionhardening' -PropertyType DWord -Value 0 -Force | Out-Null
# Allow different, but warn
New-ItemProperty -Path 'HKCU:\software\policies\microsoft\office\16.0\excel\security' -Name 'extensionhardening' -PropertyType DWord -Value 1 -Force | Out-Null
# Always match file type
New-ItemProperty -Path 'HKCU:\software\policies\microsoft\office\16.0\excel\security' -Name 'extensionhardening' -PropertyType DWord -Value 2 -Force | Out-Null