For 2013 SP1, at least, there's already a file extension andI receive the same error message when I attempt to change an existing icon. I also edited the permissions on the MMIcons folder to for application/msword
Youmodify permissions. It may wantbe necessary to double-checkchange permissions on the entire folder path.
Have you also checked to bemake sure that there isn't already something else there withthe image is roughly the same icon or mime-type that's causingfile size as the issue.one it's replacing?