后台上传图，修改修复总结

_from.php中

use backend\widgets\ActiveForm;

<?= $form->field($model, 'url')->imgInput(['style' => 'max-width:200px;max-height:200px']);?>

model 规则 

[['url'], 'file', 'skipOnEmpty' => true, 'extensions' => 'png, jpg, gif'],（或者不写）