One handy feature of the WebREPL is that it can send and get files from the board's filesystem. Tools like ampy can send and get files over the serial REPL too, but accessing files over the WebREPL can sometimes be more convenient.
Be aware file access using the WebREPL is very simple and doesn't allow uploading to specific locations on the filesystem (everything is uploaded to the root). You'll need to use other tools to put files in different locations.
To use the file features of the WebREPL first connect to the board's WebREPL. Then notice the file operation block on the right of the WebREPL:
To upload a file click Choose File and select the file. Notice the page will show you the size of the file.
Click Send to Device to initiate the upload. Once finished the file operation status should show all the data was uploaded to the board.
To download a file from the filesystem just enter the name or path to the file in the Get a file box. Then click Get from device and the file should be downloaded through the browser to your computer.