MicroPython on the BBC micro:bit has a very small filesystem. Often you’ll want to copy small files onto the device, or copy them from the micro:bit onto your own computer for further processing.
Happily, Mu has a visual way to do this by dragging and dropping files. Click on the button shown below:
The pane shown below will appear at the bottom of Mu’s window. Simply click and drag a file between the list of files on the micro:bit and those on your computer (as the animation shows). The files on your computer can be found in the folder Mu uses as your working directory (see the tutorial called where are my files?).
If you click the "Files" button while the micro:bit is still reacting to instructions from Mu (such as listing files, getting the content of a file, and so on), then you may find that afterwards the REPL behaves strangely.
The solution is simply to press restart on the device while the "Files" or "REPL" panes are off.
If in doubt while using the "Files" pane, just wait for Mu to confirm that whatever file related activity is happening, is reported as finished (as shown above).