On Windows, local files with the .xht extension are treated as text/html when they are supposed to be equivalent to application/xhtml+xml or the .xhtml extension.
Confirmed in Webkit nightly r25282
This sounds like a CFNetwork issue that needs to be tracked and fixed by Apple internally.
<rdar://problem/5710167>
This will continue to be tracked by Apple internally, closing as INVALID. Thank you for reporting this issue!