File validator adjusted

File validator mime - types added, also image validator adjusted. This PR is related with this [issue](https://github.com/yiisoft/yii2/issues/3743).
Should we also drop `types` here or not ? As for me i almost always use `mimeTypes` , but for other developers it can be annoing to specify all mimeTypes that can be matched by file extensions, so they will like to make simple `types`. 
However i think `types` can be adjusted with mime-types check, so we will compare pathinfo extension + extension that is determined on mime - type, what do you think ? If so, then we will need some additional method in `BaseFileHelper` called `getExtensionByMimeType`, simple `return isset()` .