Unfortunately it’s one of those situations where the end user is saving an *.xlsx frequently to a network location so the suggstion to save as html isn’t an option.
Well it will not work in an iframe, unless perhaps you’d install Excel on a PC with Xibo.
Basically if it’s not installed, Xibo has no idea how to open such file, if it would be installed, then I’d imagine it would ask you to open it in excel, which could not be exactly what you want.
If however, your ‘end user’ would save it to the very same location on your network but as a html, then Xibo will know how to open it (using IE11 via browser emulation).
Alternatively you can use google spreadsheet (and share it publicly) or office live (and share it publicly) then you’d have embed code that you could put in Xibo - I think that saving it as html on your network would be better though.