ImageManager dialog is using RadFileExplorer control to provide the file browsing functionality. In order to apply custom file filtering you need to subclass the content provider and orverride the ResolveDirecotry() method to filter the listed files. Please take a look at the following live demo for an example on how to apply file filtering for the explorer control:
FileExplorer / Filter files and download
An example of how to assign custom content provider to the ImageManager's dialog is available in this live demo.
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