diff --git a/server.py b/server.py
index a23d5fa52..b0dd33828 100644
--- a/server.py
+++ b/server.py
@@ -219,12 +219,24 @@ class PromptServer():
                 if os.path.isfile(file):
                     if 'preview' in request.rel_url.query:
                         with Image.open(file) as img:
-                            img = img.convert("RGB")  # jpeg doesn't support RGBA
+                            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]
+
+                            quality = 90
+                            if preview_info[-1].isdigit():
+                                quality = int(preview_info[-1])
+
                             buffer = BytesIO()
-                            img.save(buffer, format=request.rel_url.query['preview'], optimize=True, quality=90)
+                            img.save(buffer, format=image_format, optimize=True, quality=quality)
                             buffer.seek(0)
 
-                            return web.Response(body=buffer.read(), content_type=f'image/{request.rel_url.query}',
+                            return web.Response(body=buffer.read(), content_type=f'image/{image_format}',
                                                 headers={"Content-Disposition": f"filename=\"{filename}\""})
 
                     if 'channel' not in request.rel_url.query:
diff --git a/web/scripts/app.js b/web/scripts/app.js
index 7d49d5e75..ca6fc4425 100644
--- a/web/scripts/app.js
+++ b/web/scripts/app.js
@@ -54,7 +54,9 @@ export class ComfyApp {
 		 * file format for preview
 		 * @type {string}
 		 */
-		this.preview_format = "webp";
+		this.preview_format = "webp";	// L;webp;50 -> grayscale, webp, quality 50
+										// jpeg;80 -> rgb, jpeg, quality 80
+										// png -> rgb, png, default quality(=90)
 	}
 
 	static isImageNode(node) {