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Put the code before the closing of the configuration tag, as in the image below. Make sure not to place it inside any other tag. If web.config already contains an appSettings section, add the keys to the existing section. Do not create another appSettings section. You can also have keys that set ExcelTemplate and WordTemplate properties in this appSettings section.
Note: The keys are case sensitive so make sure True is capitalized. The default value of these properties is False.