Raw cells don't match nbformat specification

According [format documentation](https://nbformat.readthedocs.io/en/latest/format_description.html#code-cells) raw cells have have a following structure:
````json
{
"cell_type" : "raw",
"metadata" : {
# the mime-type of the target nbconvert format.
# nbconvert to formats other than this will exclude this cell.
"format" : "mime/type"
},
"source" : "[some nbformat output text]"
}
````
But in my Jupyter v5.7.8 I found, that my raw cells have a following structure
````json
{
"cell_type": "raw",
"metadata": {
"raw_mimetype": "text/latex"
},
"source": [
"\\newpage"
]
},
````
Is nbformat changed (without documentation updation)? Or Jupyter don't support the format the properly?

Start with the linked raw-cell section of the format documentation and compare its metadata and source representation with the Jupyter v5.7.8 example. Determine whether the specification or notebook behavior is incorrect; done when the documented format and generated raw-cell structure are reconciled.

Written by the indexing model from the issue text.