diff --git a/ClosedXML/Excel/XLWorkbook.cs b/ClosedXML/Excel/XLWorkbook.cs
index 8eaf483..a33adcc 100644
--- a/ClosedXML/Excel/XLWorkbook.cs
+++ b/ClosedXML/Excel/XLWorkbook.cs
@@ -502,15 +502,23 @@
         private static SpreadsheetDocumentType GetSpreadsheetDocumentType(string filePath)
         {
             var extension = Path.GetExtension(filePath);
-
             if (extension == null) throw new Exception("Empty extension is not supported.");
+            extension = extension.Substring(1).ToLowerInvariant();
 
-            if (extension.ToLowerInvariant().Equals(".xlsm")) return SpreadsheetDocumentType.MacroEnabledWorkbook;
+            switch (extension)
+            {
+                case "xlsm":
+                case "xltm":
+                    return SpreadsheetDocumentType.MacroEnabledWorkbook;
+                case "xlsx":
+                case "xltx":
+                    return SpreadsheetDocumentType.Workbook;
+                default:
+                    throw new ArgumentException(String.Format("Extension '{0}' is not supported. Supported extensions are '.xlsx', '.xslm', '.xltx' and '.xltm'.", extension));
 
-            if (extension.ToLowerInvariant().Equals(".xlsx")) return SpreadsheetDocumentType.Workbook;
-
-            throw new Exception(String.Format("Extension '{0}' is not supported. Supported extensions are '.xlsx' and '.xslm'.", extension));
+            }
         }
+
         private void checkForWorksheetsPresent()
         {
             if (Worksheets.Count() == 0)