This generally happens if you have saved a composition when using Firefox as a browser.
(Tip - use Chrome if you can...saves having to do this)
On a Mac, find the file and click once on the name. The name will be highlighted so you can change the filename.
Then add .json to the end of the filename. You will be asked - "Are you sure you want to change the extension from “.txt” to “.json”? "- click 'use .json.
NB .json needs to be lower case.
On a PC
This technique (as above) also works on a PC or
Find the file and click Rename File and then add .json to the end of the filename.
