lowercase when checking file name extensions.

go/images/preprocess.go

@@ -2,6 +2,7 @@ package images
 
 import (
 	"path/filepath"
+	"strings"
 )
 
 /*
@@ -14,6 +15,7 @@ import (
  */
 func MaybePreprocessImage(filename string, data []byte, width, height int) (resized []byte, w int, h int) {
 	ext := filepath.Ext(filename)
+	ext = strings.ToLower(ext)
 	switch ext {
 	case ".png", ".gif":
 		return Resized(ext, data, width, height)