rem: messenger.openAttachment(this.contentType, this.url, encodeURIComponent(this.displayName), this.uri, this.isExternalAttachment); 
add: var webNavigationInfo = Components.classes["@mozilla.org/webnavigation-info;1"] .getService(Components.interfaces.nsIWebNavigationInfo);  if (webNavigationInfo.isTypeSupported(this.contentType, null)) openAsExternal(this.url); else { messenger.openAttachment(this.contentType, this.url, encodeURIComponent(this.displayName), this.uri, this.isExternalAttachment); } 
context: createNewAttachmentInfo.prototype.openAttachment = function openAttachment(){  messenger.openAttachment(this.contentType,                            this.url,                            encodeURIComponent(this.displayName),                            this.uri,                           this.isExternalAttachment);}
meta: 10234	/local1/tlutelli/issta_data/temp/all_js/javascript/2007_temp/2007/10234/b4500b77566e1ac8acaeca1884e6d7ae7c16b6a2/msgHdrViewOverlay.js/clean/mailnews/base/resources/content/msgHdrViewOverlay.js