From FCKeditor Docs

ShowDropDialog

This feature is dependent on the ForcePastAsPlainText setting that, if true, must ensure that all pastings in the editor must be in plain text. In that case, if ShowDropDialog is set to true, the editor will also show the paste dialog on "Drag and Drop" operations to ensure that the dropped text will be plain text. Sytax of the option:

FCKConfig.ShowDropDialog = true ;