To get Debian to communicate with your Android phone so that you can download/backup your photos, videos, documents, etc onto your computer, you need to install a couple of packages. Open a Terminal and type:
sudo apt-get install jmtpfs gvfs-backends
Make sure when you connect your phone to your PC that (on your phone) you choose Media device (MTP), not Carera (PTP).
You should now be able to access media on the device by just clicking it on Thunar (file manager).
Don’t forget to Unmount/Eject the drive in Thunar BEFORE removing the USB cable, in the same way you would do with a USB stick.