Currently in the expression editor we cannot add the content type to the expression. It could be added to the fields section so we can use its value inside the expression.
It looks like the editor currently only shows the manually added field and the most common default columns of the lists. What I am also missing for example is the "FileRef" column inside document libraries which can also be handy sometimes. So I suggest to add more of the most common fields (that are already inside the default list templates) for selection inside the Expression Editor, at least:
- FileRef (only in document libraries)