Why does Yii file validator requires extensions without a dot?

HTML5’s input field for file upload requires accepted file extensions to be provided with a dot, for example accept=".jpg,.gif", but Yii’s file validator requires extensions to be provided without a dot such as extensions=>“jpg, gif”. For sake of consistency and reducing maintenance I would suggest Yii following the same approach as HTML standard recommends.

HTML5 allows not extensions but “file type specifier”: mix of dot prepended extensions and MIME types. They add dot just to figure out is it extension or Is it MIME type.
In Yii2 validator these fields are separated, so there is not need to use such crutches and I don’t understand why is it inconvenient to write file extensions to field “extensions”? In HTML it’s called not “extensions” nor “mime-types”, but “accept”.
More importantly, what do you mean by “reducing maintenance”?