diff --git a/internal/client_channel.go b/internal/client_channel.go
index 191f6da..b77914f 100644
--- a/internal/client_channel.go
+++ b/internal/client_channel.go
@@ -218,7 +218,7 @@ func (t *TClient) getExtByFilenameAndMimeType(filename, mimeType string) string
 		ext = exts[len(exts)-1]
 	}
 
-	return ext
+	return strings.ToLower(ext)
 }
 
 // saveDocument