When we upload a file, and select one image to be the 'thumbnail', after we publish, the thumbnail image we had uploaded will not be active. But will be visible if we edit the content.

Open TeamRoom

Create a TeamRoom with the attribute "Files"

Run CronJob for eZTeamRoom "Frequent"

Upload a file into the "Files" Folder in eZTeamRoom

Choose a thumbnail for the file, and select a file from your local computer

Publish and see that the thumbnail is not the one you had just uploaded.