Based on the documentation for the software being used by this site it seems that the ability to upload files is disabled by default, if the administrators decided to enable that functionality/plugin then a related button should appear within the editor's action bar.
Currently the suggested workaround is to upload the file you wish to share to a File Hosting Site (like MEGA, G-Drive, etc..) of your choosing, and then to paste a link to that file within your comment.