Could you verify that the folder, in which you are trying to upload files, is set as a value to the FileExplorer's UploadPaths property? Another possible reason for such a behavior could be if you do not have enough permissions to create/upload files into the desired folder. You can see how to test the permissions here: Troubleshooting
If none of the above is the case, I will need more detailed information in order to be able to provide more to the point help. Could you prepare a sample fully runnable project
reproducing the issue, including the used custom content provider?
Looking forward to hearing from you,
the Telerik team
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