[Peek]Fix IsDevFilePreview and white flash (#28734)

src/modules/peek/Peek.FilePreviewer/Previewers/WebBrowserPreviewer/WebBrowserPreviewer.cs

@@ -118,6 +118,8 @@ namespace Peek.FilePreviewer.Previewers
                     }
                     else
                     {
+                        // Simple html file to preview. Shouldn't do things like enabling scripts or using a virtual mapped directory.
+                        IsDevFilePreview = false;
                         Preview = new Uri(File.Path);
                     }
                 });