Add text/x-yaml MIME type.

src/yaml/yaml.contribution.ts

@@ -9,6 +9,6 @@ registerLanguage({
 	id: 'yaml',
 	extensions: ['.yaml', '.yml'],
 	aliases: ['YAML', 'yaml', 'YML', 'yml'],
-	mimetypes: ['application/x-yaml'],
+	mimetypes: ['application/x-yaml', 'text/x-yaml'],
 	loader: () => import('./yaml')
 });