imagePane.js
function (kb, x, contentTypes) {
      var cts = kb.fetcher.getHeader(x, 'content-type')
      if (cts) {
        for (var j = 0; j < cts.length; j++) {
          for (var k = 0; k < contentTypes.length; k++) {
            if (cts[j].indexOf(contentTypes[k]) >= 0) {
              return true
            }
          }
        }
      }
      return false
    }
function (kb, x, contentTypes) {
      var cts = kb.fetcher.getHeader(x, 'content-type')
      if (cts) {
        for (var j = 0; j < cts.length; j++) {
          for (var k = 0; k < contentTypes.length; k++) {
            if (cts[j].indexOf(contentTypes[k]) >= 0) {
              return true
            }
          }
        }
      }
      return false
    }
See also the source pane, which has lower precedence.