public/Get-RecipeImageFilePath.ps1

function Get-RecipeImageFilePath {
    [CmdletBinding()]
    param(
        [Parameter(Mandatory)]
        [int]$RecipeId,

        [string]$Extension = ".jpg"
    )

    if (-not $Extension.StartsWith(".")) {
        $Extension = ".$Extension"
    }

    $folder = Get-RecipeImageFolder
    Join-Path $folder ("Recipe_{0}{1}" -f $RecipeId, $Extension.ToLower())
}