1e928c53_nsIChannel___Mozilla___MDN__2
[0] contentCharset [1] ACString [2] 
The character set of the channel's content if available and if applicable. This attribute only applies to textual data. Setting contentCharset before the channel has been opened provides a hint to the channel on what the charset should be. The behavior is similar to contentType. The value of the contentCharset attribute is a mixed case string.
nsIChannel - Mozilla | MDN