Saving / Opening
Opening:
Either use thrid party file explorer and open files with the app or use the app to open files - main thing to consider is if you don't use text editor and choose to open it as text, app will import a file, that means: Every supported image gets imported as texture/material to a project, if it doesn't recognize file extension it will open it as text file (please note that only standard character set is supported and you won't able to see country/region specific characters)
There are 3 ways to open files in app:
Main way is to use internal file explorer and double click on desired file, to preview an image long tap on it it will load preview
Use not finished file explorer (if you don't use mouse to scroll you either use scrollbar or use two fingers and then move them) If you happen to use that explorer be in mind that long tapping on item switches between select mode.
With text editor (in navigation panel) this is somewhat a look of android studio -> find a file you want to open and open it by double clicking on it
Saving: (Ctrl + S)
For standard file explorer
You choose the path and then at the bottom there is a text field where you specify a filename (extension will be added automatically if not specified) or you can double click on existing file and overwrite it