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, 4 deletions

diff --git a/picture_test.go b/picture_test.go
index 5bf139d..8da7c3d 100644
--- a/picture_test.go
+++ b/picture_test.go
@@ -1,19 +1,18 @@
 package excelize
 
 import (
+	"fmt"
 	_ "image/gif"
 	_ "image/jpeg"
 	_ "image/png"
-
-	_ "golang.org/x/image/tiff"
-
-	"fmt"
 	"io/ioutil"
 	"os"
 	"path/filepath"
 	"strings"
 	"testing"
 
+	_ "golang.org/x/image/tiff"
+
 	"github.com/stretchr/testify/assert"
 )