Please consider updating the Default Accepted Media Types for file uploads.
Users get really turned off when they try to share a simple Excel document and get rejected.
This is the default value:
This is the value from open.rocket.chat:
Which allows for Excel files and more.
What else should be added to the list and made default?
http://checkfiletype.com/ may be a good place to look.
[ thanks to @bradley.hilton for helping track this down ]