Redmine

Hello,
I'm not sure if this is a defect or a configuration issue in redmine. See redmine/plugins versions below

When a user clicks on a a issue with file attached, then the file name ... Redmine is showing a screen with "no preview available" and a link to download the file. I noticed in previous versions there was a preview, so ..

• the preview on attached files is controlled by a redmine setting?
• is the preview option dependent on the file-type?

How can I get redmine to show a preview of the attachment?

Thanks in advance.
-Alex

Environment:
Redmine version 3.4.4.stable
Ruby version 2.3.4-p301 (2017-03-30) [x86_64-linux]
Rails version 4.2.8
Environment production
Database adapter Mysql2
SCM:
Git 1.7.1
Filesystem
Redmine plugins:
advanced_roadmap_v2 2.4.2
easy_baseline 1.4
easy_gantt 1.10
easy_gantt_pro 1.10
easy_wbs 2016-0.0
redmine_agile 1.4.5
redmine_checklists 3.1.5
redmine_ckeditor 1.1.4
redmine_contacts 4.1.1
redmine_contacts_helpdesk 3.0.9
redmine_dmsf 1.6.0
redmine_drawio 0.8.1
redmine_people 1.3.0