It has been noticed in Qlik Sense 2.1.1 that importing an extension on the Qlik Sense server sometimes fails, producing the error message shown in the screenshot. This can happen regardless of the extension type (visualization, mashup, template, mashup-template).
It is very likely that this is caused by a degrade in 2.1.1 that has the effect of not allowing certain file types to be uploaded. The server’s log file does not indicate why this happened.
The attached document provides several solutions to fix this problem.