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You should be able to upload such file format to discussion without any problem, just make sure that the xls. is allowed file type in Joomla's user manager.
When I go to create new discussion, under insert it says Image, Link, Special Character. There is no option to upload files, attach files. I have looked under the text box as well. I have added the xlsx in the media manager options.
First of all make sure that file uploading is allowed in group discussions, so please go to back-end>components>jomsocial>groups>groups settings and make sure that "Enable File Sharing for Discussion " is set to "yes". Also in group edit on front-end make sure that file sharing is enabled in groups.