Apple has released a Knowledge Base article on a problem with BDMV files (Blu-Ray) Seems that people were expecting to be able to open the file with QuickTime Player (You can't)
When sharing to Blu-ray from Final Cut Pro X, Motion 5, or Compressor 4, the Blu-ray (BDMV) folder on the resulting Blu-ray disc appears as a QuickTime file and it will not open. This occurs in OS X v10.8 Mountain Lion. Any Blu-ray disc (purchased, burned, or disc image) will appear this way.
The BDMV folder is not intented to be opened in QuickTime Player. If you need to view the contents of the BDMV folder on a Blu-ray disc:
Control click the BDMV icon
Choose Show Package Contents from the contextual menu. The contents of the BDMV folder will then be visible.