diff --git a/modules/ui_extra_networks.py b/modules/ui_extra_networks.py
index d8c311423..59d6ecc61 100644
--- a/modules/ui_extra_networks.py
+++ b/modules/ui_extra_networks.py
@@ -1,3 +1,4 @@
+import functools
 import os.path
 import urllib.parse
 from pathlib import Path
@@ -15,10 +16,16 @@ from modules.ui_components import ToolButton
 extra_pages = []
 allowed_dirs = set()
 
-allowed_preview_extensions = ["png", "jpg", "jpeg", "webp", "gif"]
-if shared.opts.samples_format not in allowed_preview_extensions:
-    allowed_preview_extensions.append(shared.opts.samples_format)
-allowed_preview_extensions_dot = ['.' + extension for extension in allowed_preview_extensions]
+default_allowed_preview_extensions = ["png", "jpg", "jpeg", "webp", "gif"]
+
+
+@functools.cache
+def allowed_preview_extensions_with_extra(extra_extensions=None):
+    return set(default_allowed_preview_extensions) | set(extra_extensions or [])
+
+
+def allowed_preview_extensions():
+    return allowed_preview_extensions_with_extra((shared.opts.samples_format, ))
 
 
 def register_page(page):
@@ -38,9 +45,9 @@ def fetch_file(filename: str = ""):
     if not any(Path(x).absolute() in Path(filename).absolute().parents for x in allowed_dirs):
         raise ValueError(f"File cannot be fetched: {filename}. Must be in one of directories registered by extra pages.")
 
-    ext = os.path.splitext(filename)[1].lower()
-    if ext not in allowed_preview_extensions_dot:
-        raise ValueError(f"File cannot be fetched: {filename}. Only png, jpg, webp, and gif.")
+    ext = os.path.splitext(filename)[1].lower()[1:]
+    if ext not in allowed_preview_extensions():
+        raise ValueError(f"File cannot be fetched: {filename}. Extensions allowed: {allowed_preview_extensions()}.")
 
     # would profit from returning 304
     return FileResponse(filename, headers={"Accept-Ranges": "bytes"})
@@ -278,7 +285,7 @@ class ExtraNetworksPage:
         Find a preview PNG for a given path (without extension) and call link_preview on it.
         """
 
-        potential_files = sum([[path + "." + ext, path + ".preview." + ext] for ext in allowed_preview_extensions], [])
+        potential_files = sum([[path + "." + ext, path + ".preview." + ext] for ext in allowed_preview_extensions()], [])
 
         for file in potential_files:
             if os.path.isfile(file):