I have used the Rename File task to do this, under Rename settings tab, you can use:
{STRING(Replace|{NEWNAME()}|oldfilename.txt|newfilename{DATEFORMAT(yyyyMMddHHmm)}.txt)}
So basically, you add your old or current file name, then on the right hand side of the pipe, give the new file name. In my case above, I am appending a date stamp to the old file name. You can also apply this same principle to other tasks, not just the rename file task. Also, I think there is an example in the documentation.
Hope this helps.