There were a couple of issues interacting to cause this:
Firstly, when LLViewerMediaImpl delayed loading the parcel media (which it normally does), it was losing track of the specified MIME type, but the autodiscovery code also wasn't being triggered. The code should now carry through both the specified MIME type and the autodiscovery flag when loading is delayed.
Second, the new media autodiscovery code might not work for some legacy parcel media content (for example, if it's stored on a server that doesn't report the correct MIME types). The code has been changed to first check whether the specified MIME type maps to a known plugin and allow that to override MIME type detection, which should allow this legacy content to keep working.
Made LLPluginProcessParent differentiate between failures launching/loading the plugin and failures after the plugin has been loaded. This allows us to handle launch failures differently, since retrying is unlikely to fix them.
Added new media event MEDIA_EVENT_PLUGIN_FAILED_LAUNCH to indicate a launch failure.
Added a case for the new event to LLViewerMediaImpl::handleMediaEvent() that sets the "failed init" flag to prevent retries.
svn merge -r 129913:131718 svn+ssh://svn.lindenlab.com/svn/linden/branches/pluginapi/pluginapi_05
Some branch shenannigans in the pluginapi_05 branch caused this to become a two-part merge.
Monroe Linden