diff --git a/cbconvert_func.go b/cbconvert_func.go
index 22fcc15..43b76b3 100644
--- a/cbconvert_func.go
+++ b/cbconvert_func.go
@@ -73,7 +73,7 @@ func isArchive(f string) bool {
 
 // isDocument checks if file is document.
 func isDocument(f string) bool {
-	var types = []string{".pdf", ".xps", ".epub", ".mobi", ".docx", ".pptx"}
+	var types = []string{".pdf", ".xps", ".epub", ".mobi", ".docx", ".pptx", ".xlsx"}
 	for _, t := range types {
 		if strings.ToLower(filepath.Ext(f)) == t {
 			return true