As you may know, WifiTransfer is an app for Ubuntu Touch that permits the users to upload files the OS, via FTP. It is open-source and can be installed from the Ubuntu Store.
The latest version available has been just released, permitting users to copy files on the phone via wifi, without having to use a cable. Also, it automatically shows in the file manager Network section, for an easier usage.
As a reminder, Ubuntu Touch is a stable operating system, but it is not mature enough, compared to Android and iOS. For now, there are three Ubuntu Phones: Bq Aquaris E5 HD, Bq Aquaris E4.5, Meizu MX4, but the system is also officially supported on the LG Nexus 4 device.