After upgrading to kiconthemes 6.3.0, attachment icons in the message view are no longer displayed.
Created attachment 170425 [details] Missing icons in message headers
Created attachment 170426 [details] Missing icons in message body
it fixed in master for sure. I need to look at if it can be backport. but packporting means breaking kmail 6.1.0 with framework < 6.3.0... So it's not really cool...
by default pim 6.1.0 will release against kf <6.3 so not sure that it's a good idea to break it now.