How to use facets transform mime_types

I'm looking into the way on how to change the mime_type field into a more readable format.

Following the documentation: Content sources API reference | Workplace Search Guide [8.0] | Elastic

I tried the following:

curl \
--request "PUT" \
--url "https://XXXX.ent.eu-central-1.aws.cloud.es.io/api/ws/v1/sources/XXX" \
--header "Authorization: Bearer XXXX" \
--header "Content-Type: application/json" \
--data '
{
    "name": "GoogleDrive",
    "is_searchable": true,
    "facets": {
        "overrides": [
            {
                "field": "mime_type",
                "name": "Mime_type_clean",
                "enabled": true,
                "transform": "mime_type",
            }
        ]
    },
}
'

But it is not updated when I retrieve the sources again. How can I "activate" the human-readable mime_types?