A burst item contains a deliver that uses format Excel with one or more Excel Extensions. Attempt to switch the format to another type and the following error occurs:
Excel Post Process can only be applied if the delivery format is Excel or if the document is of type Excel
This issue is under investigation.
To work around the issue, first remove the extensions and Save. Then change the format.
This issue is addressed in Build 240 FP6 and InfoBurst 2021.2.
InfoBurst Platform Build 240 FP6 - Release Notes
InfoBurst 2021.2.4 - Release Notes