I use this expression with my Saving Sequential Back Ups During Development auto-back up system and it works for me:
Let ( [ F = Get ( FilePath ) ] ;
Left ( F ; Position ( F ; "." ; Length ( F ) ; -1 ) - 1 ) & " " & Substitute ( Get ( CurrentTimeStamp ) ; ["/" ; "-"] ; [":" ; " " ] ) & ".fp7"
) // Let ]
Note that it removes both : and / character's from the filename, both of which can be illegal in a file's name. It also computes a file path to the same folder as the current file and appends the time stamp data to the end of the file name, but before adding the ".fp7" file extension.