1e928c53_nsIChannel___Mozilla___MDN__2
[0] contentType [1] ACString [2] 
The MIME type of the channel's content if available. Note: The content type can often be wrongly specified (For example wrong file extension, wrong MIME type, wrong document type stored on a server and so on.), and the caller most likely wants to verify with the actual data. Setting contentType before the channel has been opened provides a hint to the channel as to what the MIME type is. The channel may ignore this hint in deciding on the actual MIME type that it will report. Setting contentType after onStartRequest has been fired or after open() is called will override the type determined by the channel. Setting contentType between the time that asyncOpen() is called and the time when onStartRequest is fired has undefined behavior at this time. The value of the contentType attribute is a lowercase string. A value assigned to this attribute will be parsed and normalized as follows: Any parameters (delimited with a ';') will be stripped. If a charset parameter is given, then its value will replace the the contentCharset attribute of the channel. The stripped contentType will be lowercased. Any implementation of nsIChannel must follow these rules.
nsIChannel - Mozilla | MDN