This is Mostlymatter binaries, a fork of Mattermost without users limit. See here for more informations.

Mostlymatter is licensed under the same terms than Mattermost. Mostlymatter’s logo is licensed under the terms of the CC-By-NC v4.0 license and is designed by Geoffrey Dorne.

To verify the authenticity of the file you downloaded, download the .minisig file as well, install minisign, then:

minisign -Vm mostlymatter-* -P RWSoFpHRlDgrwN2JmiT+JoV2nQBDzvhOgk4GGdmpzGRuhSRD4/F81JX3

If you can't install minisign, you can verify the SHA512SUM of the file. Download the .sha512 file as well, then:

sha512sum -c mostlymatter-*.sha512