If you are coming over from Yii 1.x to Yii 2, and already read this guide, you may note that namespaces are very important in Yii 2 to identify classes. But do you know the list of seeded path aliases that come shipped with a basic or advanced app? Here is my attempt to list them.
Uploading files in a webapp can be extremely tricky and sometimes the quickest way to do it is to store the file directly in the webserver or into a DNS (like Amazon S3) and then to save the link and the metadata inside a table in the DB. The thing is that you'll have to deal with file permissions, server storage, file management and so on (which is perfectly fine, by the way).