> Afaik, the type attribute (or whatever its name may be) in this case 
> means to indicate what types it accepts. So supposedly <object 
> src="bla.html" type="image/*">fallback content</object> would result in 
> the fallback content being shown, unless the server associates the .html 
> extension with an image/ type.

The problem with all the XHTML2-attribs (srctype, hreftype, hreflang)
based on http content negotiation is that 
a) servers will ignore them or handle them in unexpected ways (try get
http://www.w3.org/TR/xhtml2/  as "application/xml,application/xhtml+xml":
you'll receive "text/html")
b) they fail miserably in non-negotiating environments