Dropdown question select a defualt from entity list

[BJ Freeman]

want to modify the drop down to select a specific default of text/plain
so would i mofify no-current-selected-key="_NA_"
to
no-current-selected-key="text/plain"

        <field name="contentMimeTypeId">
            <drop-down no-current-selected-key="_NA_" allow-empty="false">
                <entity-options description="${mimeTypeId}"
entity-name="MimeType" key-field-name="mimeTypeId">
                    <entity-order-by field-name="mimeTypeId"/>
                </entity-options>
            </drop-down>
        </field>