diff --git a/ClosedXML/Excel/XLWorkbook.cs b/ClosedXML/Excel/XLWorkbook.cs
index 6fbe11c..3c63b61 100644
--- a/ClosedXML/Excel/XLWorkbook.cs
+++ b/ClosedXML/Excel/XLWorkbook.cs
@@ -474,13 +474,17 @@
             switch (extension)
             {
                 case "xlsm":
-                case "xltm":
                     return SpreadsheetDocumentType.MacroEnabledWorkbook;
 
+                case "xltm":
+                    return SpreadsheetDocumentType.MacroEnabledTemplate;
+
                 case "xlsx":
-                case "xltx":
                     return SpreadsheetDocumentType.Workbook;
 
+                case "xltx":
+                    return SpreadsheetDocumentType.Template;
+
                 default:
                     throw new ArgumentException(String.Format("Extension '{0}' is not supported. Supported extensions are '.xlsx', '.xslm', '.xltx' and '.xltm'.", extension));
             }