diff --git a/server.py b/server.py
index b0dd3382..c0b4729d 100644
--- a/server.py
+++ b/server.py
@@ -221,19 +221,18 @@ class PromptServer():
                         with Image.open(file) as img:
                             preview_info = request.rel_url.query['preview'].split(';')
 
-                            if preview_info[0] == "L" or preview_info[0] == "l":
-                                img = img.convert("L")
-                                image_format = preview_info[1]
-                            else:
-                                img = img.convert("RGB")  # jpeg doesn't support RGBA
-                                image_format = preview_info[0]
+                            image_format = preview_info[0]
+                            if image_format not in ['webp', 'jpeg']:
+                                image_format = 'webp'
 
                             quality = 90
                             if preview_info[-1].isdigit():
                                 quality = int(preview_info[-1])
 
                             buffer = BytesIO()
-                            img.save(buffer, format=image_format, optimize=True, quality=quality)
+                            if image_format in ['jpeg']:
+                                img = img.convert("RGB")
+                            img.save(buffer, format=image_format, quality=quality)
                             buffer.seek(0)
 
                             return web.Response(body=buffer.read(), content_type=f'image/{image_format}',
diff --git a/web/scripts/ui.js b/web/scripts/ui.js
index 6b764d43..a26eedec 100644
--- a/web/scripts/ui.js
+++ b/web/scripts/ui.js
@@ -465,12 +465,11 @@ export class ComfyUI {
 		/**
 		 * file format for preview
 		 *
-		 * L?;format;quality
+		 * format;quality
 		 *
 		 * ex)
-		 * L;webp;50 -> grayscale, webp, quality 50
+		 * webp;50 -> webp, quality 50
 		 * jpeg;80 -> rgb, jpeg, quality 80
-		 * png -> rgb, png, default quality(=90)
 		 *
 		 * @type {string}
 		 */