From version 6.0 there exists the database property "Use LZ1 compression for attachments" which can compress attachments better then the standart "Huffman" method. However this property is by default not enabled and it also only affects attachments which are created after it has been enabled (or disabled). This means that existing attachments are unaffected.
I have always wanted an option that the compact-task would transfer ALL existing attachments in a database according to the current database property. This would definitly save some disk space and it still is not late for such function.
Added on 6.11.2007:
I have been informed by Pumynt Chooboonraj that exactly this functionality has already been added to Domino in version 6.5.6 and 7.0.1. Here is a technote which decribes it: www-1.ibm.com/support/docview.wss
Thanks Pumynt !
And thanks Lotus for adding this function !
<rant>Oh and congratualtions, you also did a great job in hiding this function from us. Don't make a big announcement, don't put it in the 'What's new section' and also don't mention it in the help-file for compact parameters. Just add it secretly in a maintanace release, post a technote and wait if anybody finds it. :-)))</rant>
Added on 9.11.2007:
The compact parameter seems to work fine but it is (as always :-)) crippled. From the technote:
"The feature does not act on attachments that were not previously compressed and does not exist in Notes' Compact code." This actually means that attachments, which were send my mail, or which were copy-pasted from a mail database will not be compacted with this function.