I think you are missing something ...
What file was it?
The proper media-type for xhtml is "application/xhtml+xml".
In fact the "application/xml" media type tells you almost nothing about the kind of file as pure xml can represent many different file types.
And pure xml islands in the spine are been long deprecated by the epub standard.
These are the specific xml based media types recognized by Sigil and current or previous epub specifications:
Code:
"application/xhtml+xml" (.xhtml)
"application/x-dtbncx+xml" (.ncx)
"application/oebps-package+xml" (.opf)
"application/pls+xml"; (.pls)
"application/smil+xml" (.smil)
"image/svg+xml" (.svg)
"application/ttml+xml" (.ttml)
"application/oebps-page-map+xml"
"application/vnd.adobe-page-template+xml"
Note: xml and therefore xhtml are case sensitive so the media-type is as well.
For a more complete list of recognized media types see Sigil's source code here:
https://github.com/Sigil-Ebook/Sigil...MediaTypes.cpp