Well this app is called WifiTransfer, and it is easy to install.
You search for "Wifi transfer" in the Ubuntu Store
Select the app and click on install
Once installed you will be able to transfer files to and from you Desktop PC and your Phone in a flash.
When you open the app click on "Turn on WifiTransfer"
and you will be able to transfer files to and from you Desktop PC and Phone browsing the network with Nautilus or using the "connect to server" function of File Manager: