This line, because it uses the XML namespace "tools", is not reflected at runtime. It's there just to give you something to look at in the UI editor. 

The "file_size" string resource is filled in at runtime with an actual number, so if I had instead put tools:text="@file_size", the screenshot would've read: "File Size: %1$s" which wouldn't illustrate very much.

I think in the long run it's better if the file size is not given in bytes, but I wanted to make small changes before getting up in the actual code