Validate file extension in PowerShell parameter

valiateFileExtension.ps1

param (
  [ValidateScript({
      if (-Not ($_ | Test-Path) ) { throw "File does not exist" }
      if (-Not ($_ | Test-Path -PathType Leaf) ) { throw "The Path argument must be a file. Folder paths are not allowed." }
      if ($_ -notmatch "\.tfplan$") { throw "The file specified in the path argument must be type tfplan" }
      return $true
    })]
  [System.IO.FileInfo]
  $FilePath
)

Source: Validating file and folder paths in PowerShell parameters