When accessing a NAS drive via the Servers section in the StreamMagic app, you’ll usually see a "Play All" option when viewing an album or playlist. However, in the Folders section, this option may not appear.
As a general rule when it comes to UPnP browsing, the StreamMagic app acts as a simple file browser in the way that it displays content and options. The StreamMagic app relies on how the UPnP server presents its content. If the server provides directories as "UPnP Containers," the app can display options like "Play All." But if the server presents them as generic folders, this option won’t be available.
If you want to have these playback options on a given folder, then there may be options to configure this in your UPnP server settings - some servers allow you to configure how content is shared, but this will vary by server.