diff -r 7b1b88e27a20 -r 48c4eec2b7e6 wp/wp-includes/js/plupload/wp-plupload.js
--- a/wp/wp-includes/js/plupload/wp-plupload.js	Thu Sep 29 08:06:27 2022 +0200
+++ b/wp/wp-includes/js/plupload/wp-plupload.js	Fri Sep 05 18:40:08 2025 +0200
@@ -363,6 +363,11 @@
 					error( pluploadL10n.noneditable_image, {}, file, 'no-retry' );
 					up.removeFile( file );
 					return;
+				} else if ( file.type === 'image/avif' && up.settings.avif_upload_error ) {
+					// Disallow uploading of AVIF images if the server cannot edit them.
+					error( pluploadL10n.noneditable_image, {}, file, 'no-retry' );
+					up.removeFile( file );
+					return;
 				}
 
 				// Generate attributes for a new `Attachment` model.