[MM-21195] add file extension on download (#1149)
[MM-21195] add file extension on download (#1149)

* [MM-21195] select extension

* win fixes

* cleanup

mattermost/desktop,yuya-oc/electron-mattermost,yuya-oc/electron-mattermost,mattermost/desktop,mattermost/desktop,mattermost/desktop,mattermost/desktop,yuya-oc/electron-mattermost