diff --git a/packages/lib/mime-utils-types.js b/packages/lib/mime-utils-types.js
index 4a73d2a09..d5db4ab29 100644
--- a/packages/lib/mime-utils-types.js
+++ b/packages/lib/mime-utils-types.js
@@ -776,7 +776,10 @@ const mimeTypes = [
 	{ t: 'x-conference/x-cooltalk', e: ['ice'] },
 ];
 
-// Note: if the list above is ever updated, make sure Markdown doesn't appear twice
+// Note: if the list above is ever updated, make sure Markdown doesn't appear
+// twice. In general, put any change here, so that we know what differs from the
+// original list.
 mimeTypes.push({ t: 'text/markdown', e: ['md', 'markdown'] });
+mimeTypes.push({ t: 'image/avif', e: ['avif'] });
 
 module.exports = mimeTypes;
diff --git a/packages/lib/models/Resource.ts b/packages/lib/models/Resource.ts
index 9f99a6a56..6cb8083b3 100644
--- a/packages/lib/models/Resource.ts
+++ b/packages/lib/models/Resource.ts
@@ -48,7 +48,7 @@ export default class Resource extends BaseItem {
 	}
 
 	public static isSupportedImageMimeType(type: string) {
-		const imageMimeTypes = ['image/jpg', 'image/jpeg', 'image/png', 'image/gif', 'image/svg+xml', 'image/webp'];
+		const imageMimeTypes = ['image/jpg', 'image/jpeg', 'image/png', 'image/gif', 'image/svg+xml', 'image/webp', 'image/avif'];
 		return imageMimeTypes.indexOf(type.toLowerCase()) >= 0;
 	}