Use single quotes

lib/utils.js

@@ -8,17 +8,17 @@ exports.getImageMimeType = function getImageMimeType (imagePath) {
   var fileExtension = /[^.]+$/.exec(imagePath)
 
   switch (fileExtension[0]) {
-    case "bmp":
-      return "image/bmp"
-    case "gif":
-      return "image/gif"
-    case "jpg":
-    case "jpeg":
-      return "image/jpeg"
-    case "png":
-      return "image/png"
-    case "tiff":
-      return "image/tiff"
+    case 'bmp':
+      return 'image/bmp'
+    case 'gif':
+      return 'image/gif'
+    case 'jpg':
+    case 'jpeg':
+      return 'image/jpeg'
+    case 'png':
+      return 'image/png'
+    case 'tiff':
+      return 'image/tiff'
     default:
       return undefined
   }