how can a firefox extension detect content-type of the page loaded?

since my extension's pageload is triggered 开发者_StackOverflow中文版even when I view css or js files, i want to add another check that triggers my extension only when the current page's content-type is text/html .

//eg: at my page load handler
function onPageload(){

  // only want to proceed if content-type reflects a text/html or */html page
  if ( contentTypeIsHtml() ){
    //continue here
  }
}

what should contentTypeIsHtml() do ?

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You can get the content type using the document.contentType property (this is not standard DOM, but is there to be used by extensions)

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Check out the source to the add-on JSONView, unzip the xpi and take a look at components/jsonview.js, towards the end of the source the add-on registers itself for the mime-type application/json. You could probably do something similar.