Se pictures below.
Background/problem:
If a visualization file is generated for an item, it is not possible to regenerate the visualization file. If you try to regenerate the visualization file by clicking the action menu on an item and then selecting "Visualization" -> "Regenerate All" or "Regenerate One" you are getting the message shown in the picture below telling "The visualization for the selected item will not be regenerated since it already has a visualization."
Since the generation of visualization files is failing relatively frequent by generating incorrect or white thumbnails, an option for recreating these visualization files is highly needed.
Suggestion for improvement:
The "Regenerate All" or "Regenerate One" for visualization files should be available also for items with already existing visualization files.