Fix file corrupted in some cases, check file extension and format code

Fix file corrupted when save as in XLAM / XLSM / XLTM / XLTX extension in some case
New exported error ErrWorkbookExt has been added, and check file extension on save the workbook
Format source code with `gofumpt`

1 files changed, 2 insertions, 2 deletions

diff --git a/sheet.go b/sheet.go
index 2426bf8..f10ca07 100644
--- a/sheet.go
+++ b/sheet.go
@@ -368,7 +368,7 @@ func (f *File) SetActiveSheet(index int) {
 // GetActiveSheetIndex provides a function to get active sheet index of the
 // spreadsheet. If not found the active sheet will be return integer 0.
 func (f *File) GetActiveSheetIndex() (index int) {
-	var sheetID = f.getActiveSheetID()
+	sheetID := f.getActiveSheetID()
 	wb := f.workbookReader()
 	if wb != nil {
 		for idx, sheet := range wb.Sheets.Sheet {
@@ -1723,7 +1723,7 @@ func (f *File) UngroupSheets() error {
 func (f *File) InsertPageBreak(sheet, cell string) (err error) {
 	var ws *xlsxWorksheet
 	var row, col int
-	var rowBrk, colBrk = -1, -1
+	rowBrk, colBrk := -1, -1
 	if ws, err = f.workSheetReader(sheet); err != nil {
 		return
 	}