Please refer image and attached excel file for reference.
steps to reproduce
1. go to the client screen and download the template for import client by selecting the entity as PERSON
2. fill in all the mandatory columns and try to upload
The system will throw up an error message "error.msg.invalid.file.extension -"
Extention is correct and all the data inputs are correct, looks like a bug
You can test the same on https://demo.mifos.io/#/bulkimportclients
The issue is present on the staging server as well.
So the issue existing on the staging 19.12 as well as demo codebase 18.03.