What steps will reproduce the problem?
1. Kill the app while saving a big file. (Of course the real problem is 
that the system could hang while saving or the app could crash or 
whatever).


What is the expected output? What do you see instead?
If something goes wrong while saving, the old file should be retained.


Please provide any additional information below.
As far as I understand, save is done directly overwriting the old file. The 
right wat should be: open a temp file, save there, then overwrite the old 
one with the temp.