Problem Statement:
Are you facing issue on author instance where preview of DITA asset is not rendering correctly and this happens on asset details page ? Do you see the html encoding rather than actual page UI - Refer the the screenshot below.

Precondition:
Do you have dispatcher set up on your author instance? If yes, the below solution can help you in resolving this issue.
Remedy:
It could be because of the missing configuration. You should need to enable the mime type for DITA files.
Here are the additional details:
Ditamap and Dita topics shown in Assets UI have to be rendered with Content-Type “text/html”. So to have this work correctly in case a Dispatcher is setup in front of Author, please do following:
- Connect server via ssh and browse to httpd.conf and check the path for file “mime.types” file (it could be in /etc/mime.types or /conf/mime.types)
- Edit mime.types file and
- Search for “text/html” the default value string for this will be “html htm”
- Update this string to “html htm ditamap dita”
This will make sure that the ditamap and dita topics are rendered as HTML in Assets UI.