nixos/roundcube: adjust maxAttachmentSize, clarify documentation

nixos/doc/manual/release-notes/rl-2411.section.md

@@ -174,6 +174,8 @@
 
 - The `services.patroni.raft` option has been removed, as Raft has been [deprecated by upstream since 3.0.0](https://github.com/patroni/patroni/blob/master/docs/releases.rst#version-300)
 
+- `services.roundcube.maxAttachmentSize` will multiply the value set with `1.37` to offset overhead introduced by the base64 encoding applied to attachments.
+
 ## Other Notable Changes {#sec-release-24.11-notable-changes}
 
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nixos/modules/services/mail/roundcube.nix

@@ -93,13 +93,17 @@ in
     maxAttachmentSize = mkOption {
       type = types.int;
       default = 18;
+      apply = configuredMaxAttachmentSize: "${toString (configuredMaxAttachmentSize * 1.37)}M";
       description = ''
         The maximum attachment size in MB.
-
-        Note: Since roundcube only uses 70% of max upload values configured in php
-        30% is added automatically to [](#opt-services.roundcube.maxAttachmentSize).
+        [upstream issue comment]: https://github.com/roundcube/roundcubemail/issues/7979#issuecomment-808879209
+        ::: {.note}
+        Since there is some overhead in base64 encoding applied to attachments, + 37% will be added
+        to the value set in this option in order to offset the overhead. For example, setting
+        `maxAttachmentSize` to `100` would result in `137M` being the real value in the configuration.
+        See [upstream issue comment] for more details on the motivations behind this.
+        :::
       '';
-      apply = configuredMaxAttachmentSize: "${toString (configuredMaxAttachmentSize * 1.3)}M";
     };
 
     configureNginx = lib.mkOption {