Steps to reproduce:

1. Navigate to Administration -> Media types -> Create media type
2. Create media type with following data:
   1. Type: Webhook
   2. Script: Please put any js code.
   3. Parameters: Let the default parameters will stay.
   4. Press "Add" button to save a mediatype
3. Click to edit the created media type.
4. Change the type from "Webhook" to "SMS", add some text to "GSM modem" and press "Update".
5. Make API request to method mediatype.get specifying the ID of created media type.

{
    "jsonrpc": "2.0",
    "method": "mediatype.get",
    "params": {
        "output": "extend",
       "mediatypeids": "<your media type id>"
    },
    "auth": "<your auth key>",
    "id": 1
}

Result:
After changing the type, the response of mediatype.get still contains the parameters for webhook.

Expected:
Parameters field contains empty array after media type update with changed type from "Webhook" to any other type.