You are downloading the Tartan AppX file v126.96.36.19972 for Windows Phone. Tartan is a free and useful Social app: Simple Slack messaging client for Windows Phone 8.1 and later.This is not the official Slack app, many people think it's better ... You can now download the offline AppX file to install & try it.
Download Locations for Tartan 188.8.131.5272 AppX file
Important Notice: Now Due To Unspecified Limitations, You Can Not Use The Offline AppX File To Install It Any More.
To try to install it manually from an SD card, you should choose one of the server location above to get the offline Tartan AppX file, move the file to your phone's SD card and then tap 'Install local apps' in the phone's App list.