function Show-SelectFileDialog

function Show-SelectFileDialog
    {
    [CmdletBinding()]

    Param
        (
        [Parameter (
            Mandatory,
            Position = 0
            )]
            [ValidateNotNullOrEmpty()]
            [string]
            $TopDir,

        # extension examples = "*", "*.*", "log", ".txt", "*.csv"
        [Parameter (
            Position = 1
            )]
            [string]
            $FileType = '*.*'
        )

    switch ($FileType)
        {
        # extensionless files can't be shown on their own
        #    so they are included with the ALL files group
        {$_ -in ('.', '*.', '', '*', '.*', '*.*')}
            {
            $FilterString = "All files (*.*)|*.*"
            break
            }
        {$_.StartsWith('*.')}
            {
            $TypeName = $_.TrimStart('*.').ToUpper()
            $FilterString = "$TypeName files ($FileType)|$FileType|All files (*.*)|*.*"
            break
            }
        {$_.StartsWith('.')}
            {
            $TypeName = $_.TrimStart('.').ToUpper()
            $FilterString = "$TypeName files (*$FileType)|*$FileType|All files (*.*)|*.*"
            break
            }
        default
            {
            $TypeName = $_.ToUpper()
            $FilterString = "$TypeName files (*.$FileType)|*.$FileType|All files (*.*)|*.*"
            }
        }

    [System.Reflection.Assembly]::LoadWithPartialName("System.Windows.Forms") |
        Out-Null
    $SelectFileDialog = New-Object System.Windows.Forms.OpenFileDialog
    $SelectFileDialog.InitialDirectory = $TopDir
    $SelectFileDialog.Filter = $FilterString
    $SelectFileDialog.Title = 'Please select a file and then click [Open]'

    # finally show the dialog
    $SelectFileDialog.ShowDialog() |
        Out-Null

    # send the filename to output
    $SelectFileDialog.FileName
    }