Redmine displays thumbnails of attachments when the setting "Display attachment thumbnails" is enabled, but it is turned off by default. So, some users may not know the feature. in my observation, almost half of admins unaware of this feature.

I suggest turning on the feature by default. It improves convenience and I think there are not many use cases that thumbnails should not be displayed. Even if you don't want to display thumbnails, you can easily turn off the setting in the Administration screen.

The feature of displaying thumbnails is a common feature in many applications nowadays. It is a pity if users misunderstand that Redmine lucks the feature.

diff --git a/config/settings.yml b/config/settings.yml
index d345a4cfa..e52a4e517 100644
--- a/config/settings.yml
+++ b/config/settings.yml
@@ -298,7 +298,7 @@ default_notification_option:
 emails_header:
   default: ''
 thumbnails_enabled:
-  default: 0
+  default: 1
 thumbnails_size:
   format: int
   default: 100