Updates asset alt text to use getThumbAlt

CHANGELOG-WIP.md

@@ -7,6 +7,8 @@
 - Element index pages now have visually-hidden “Sources” headings for screen readers. ([#12961](https://github.com/craftcms/cms/pull/12961))
 - Element metadata fields now have visually-hidden “Metadata” headings for screen readers. ([#12961](https://github.com/craftcms/cms/pull/12961))
 - Structure elements within element indexes now convey their levels to screen readers. ([#13020](https://github.com/craftcms/cms/pull/13020))
+- Image asset thumbnails in the control panel no longer have `alt` attributes if the asset is missing an Alternative Text value. ([#12724](https://github.com/craftcms/cms/pull/12724))
+- Non-image asset thumbnails in the control panel now have `alt` attributes set to the file extension. ([#12724](https://github.com/craftcms/cms/pull/12724))
 
 ### Administration
 

src/elements/Asset.php

@@ -1533,7 +1533,7 @@ public function getImg(mixed $transform = null, ?array $sizes = null): ?Markup
                 'width' => $this->getWidth(),
                 'height' => $this->getHeight(),
                 'srcset' => $sizes ? $this->getSrcset($sizes) : false,
-                'alt' => $this->alt ?? false,
+                'alt' => $this->getThumbAlt(),
             ]);
         } else {
             $img = null;
@@ -1957,6 +1957,11 @@ public function getThumbAlt(): ?string
             return null;
         }
 
+        $extension = $this->getExtension();
+        if (!Image::canManipulateAsImage($extension)) {
+            return $extension;
+        }
+
         return $this->alt;
     }
 
@@ -1998,7 +2003,7 @@ public function getPreviewThumbImg(int $desiredWidth, int $desiredHeight): strin
         return Html::tag('img', '', [
             'sizes' => "{$thumbSizes[0][0]}px",
             'srcset' => implode(', ', $srcsets),
-            'alt' => $this->alt ?? $this->title,
+            'alt' => $this->getThumbAlt(),
         ]);
     }
 

src/web/assets/cp/src/js/ElementThumbLoader.js

@@ -129,7 +129,7 @@ Craft.ElementThumbLoader.Worker = Garnish.Base.extend({
     var $img = $('<img/>', {
       sizes: $container.attr('data-sizes'),
       srcset: $container.attr('data-srcset'),
-      alt: $container.attr('data-alt') || '',
+      alt: $container.attr('data-alt') || null,
     });
     this.addListener($img, 'load,error', 'loadNext');
     $img.appendTo($container);