differences_in_mastoapi_responses.md: `content_type` parameter of post submission

docs/api/differences_in_mastoapi_responses.md

@@ -58,3 +58,4 @@ Has these additional fields under the `pleroma` object:
 Additional parameters can be added to the JSON body/Form data:
 
 - `preview`: boolean, if set to `true` the post won't be actually posted, but the status entitiy would still be rendered back. This could be useful for previewing rich text/custom emoji, for example.
+- `content_type`: string, contain the MIME type of the status, it is transformed into HTML by the backend. You can get the list of the supported MIME types with the nodeinfo endpoint.