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, 3 insertions, 0 deletions

diff --git a/errors.go b/errors.go
index 8368fee..ebbcef6 100644
--- a/errors.go
+++ b/errors.go
@@ -106,6 +106,9 @@ var (
 	// ErrImgExt defined the error message on receive an unsupported image
 	// extension.
 	ErrImgExt = errors.New("unsupported image extension")
+	// ErrWorkbookExt defined the error message on receive an unsupported
+	// workbook extension.
+	ErrWorkbookExt = errors.New("unsupported workbook extension")
 	// ErrMaxFileNameLength defined the error message on receive the file name
 	// length overflow.
 	ErrMaxFileNameLength = errors.New("file name length exceeds maximum limit")