The mimetype property of a KTextEditor::Document object is not fixed for its lifetime, at least in the implementation of KTextEditor::DocumentPrivate.

In that code it is depending on the outcome of the queries to QMimeDatabase, which itself depends on the current initial 4k bytes and/or the path of the url which is set to the document. So the mimetype can be different over the lifetime of the document instance (e.g. when saving under another filename or when editing a new file and switching between Markdown and Asciidoc during initial work).

Watchers of a document instance, like the preview plugin, could be happy to be informed about changes of the mimetype, so they can adapt to it.