I just checked the VSSX file on our server and it appears to be corrupted. I've sent the correct file over to the webteam for them to re-upload.
I'll let you know when that's done. If they update it and you happen to check it before I respond, the MD5 of the correct file is:
code:caa2cc09cd5a8cca3f656267470e2cbe SummitX-G2-04142016.vssx
In the meantime, I've copied the correct file to our FTP server.
http://tech.extremenetworks.com/ftp/ftp_new.aspx?file=Visio/SummitX-G2-04142016.vssx
If you need the VSS (not VSSX) file, I'll be happy to convert it for you. Just let me know.