+1 We have the same issue with IE when downloading from SharePoint 2010. Seems to be okay in Chrome, FF, Safari. Once downloaded to desktop, it's recognized as a twbx.
Jason, twbx files are actually zip files. Tableau just gives them a 'special' extension. You can open/extract a twbx file with any zip utility. Your home computer is recognizing the twbx file as the zip file it is; that extension has been associated with your zip utility (or sharepoint is changing the file extension to 'zip' because it recognizes what the file really is). I don't use sharepoint so I don't have any suggestions on how to smooth out this friction. Just wanted to let you know what's probably going on.