Code:
Option Explicit
Sub Macro1()
    Select Case ActiveWorkbook.FileFormat
      Case xlAddIn
          MsgBox "AddIn"
      Case xlCSV
          MsgBox "CSV"
      Case xlCSVMac
          MsgBox "CSVMac"
      Case xlCSVMSDOS
          MsgBox "CSVMSDOS"
      Case xlCSVWindows
          MsgBox "CSVWindows"
      Case xlDBF2
          MsgBox "DBF2"
      Case xlDBF3
          MsgBox "DBF3"
      Case xlDBF4
          MsgBox "DBF4"
      Case xlDIF
          MsgBox "DIF"
      Case xlExcel2
          MsgBox "Excel2"
      Case xlExcel2FarEast
          MsgBox "Excel2FarEast"
      Case xlExcel3
          MsgBox "Excel3"
      Case xlExcel4
          MsgBox "Excel4"
      Case xlExcel5
          MsgBox "Excel5"
      Case xlExcel4Workbook
          MsgBox "Excel4Workbook"
      Case xlIntlAddIn
          MsgBox "IntlAddIn"
      Case xlIntlMacro
          MsgBox "IntlMacro"
      Case xlWorkbookNormal
          MsgBox "WorkbookNormal"
      Case xlSYLK
          MsgBox "SYLK  "
      Case xlTemplate
          MsgBox "Template"
      Case xlCurrentPlatformText
          MsgBox "CurrentPlatformText"
      Case xlTextMac
          MsgBox "TextMac"
      Case xlTextMSDOS
          MsgBox "TextMSDOS"
      Case xlTextPrinter
          MsgBox "TextPrinter"
      Case xlTextWindows
          MsgBox "TextWindows"
      Case xlWJ2WD1
          MsgBox "WJ2WD1"
      Case xlWK1
          MsgBox "WK1"
      Case xlWK1ALL
          MsgBox "WK1ALL"
      Case xlWK1FMT
          MsgBox "WK1FMT"
      Case xlWK3
          MsgBox "WK3"
      Case xlWK4
          MsgBox "WK4"
      Case xlWK3FM3
          MsgBox "WK3FM3"
      Case xlWKS
          MsgBox "WKS"
      Case xlWorks2FarEast
          MsgBox "Works2FarEast"
      Case xlWQ1
          MsgBox "WQ1"
      Case xlWJ3
          MsgBox "WJ3"
      Case xlWJ3FJ3
          MsgBox "WJ3FJ3"
    End Select
End Sub