The content area of RadEditor is a standard editable iframe / div so if you are able to implement your scenario in an editable div you will be able to implement it in RadEditor.
If you want to preview the flash player in the media manager, you should customize the MediaManager dialog or create your own custom dialog
from scratch. You can put RadFileExplorer in it for a File Browser if needed.
If you need to modify the content inserted from the MediaManager, you can attach to the OnClientPasteHtml
The custom content filters are executed only when switching to HTML mode or submitting the content so I don't think that they will be helpful to displaying the flash player in RadEditor.
Note that we do not support third party controls and players and this is an entirely custom scenario, it should be implemented by the developer working with the control.
the Telerik team
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