Content sniffer interface. Components implementing this interface can
determine a MIME type from a chunk of bytes.
Given a chunk of data, determines a MIME type. Information from the given
request may be used in order to make a better decision.
@throw NS_ERROR_NOT_AVAILABLE if no MIME type could be determined.
@note Implementations should consider the request read-only. Especially,
they should not attempt to set the content type property that subclasses of
nsIRequest might offer.
| aRequest | The request where this data came from. May be null. |
| aData | Data to check |
| aLength | Length of the data |
| The content type |