Media Youtube/Vimeo is being saved (or rendered) incorrectly as application/octet-stream
I was having the same problem. The YouTube video was added correctly using the Media browser, the thumbnail was generated and it was even saved in the database correctly prior to saving the node.

After the node was saved, the entry in the database changed to application/octet-stream with a filename of the description twice, as well as a URI of public://

Something was altering the file details when the node was being saved. There weren't many options I could think of, and a quick review of the field settings revealed filefield_paths was enabled for the field, which was altering the filename.

I disabled fielfield_paths for the field and saved the node again, which resulted in the video displaying properly.

If you have filefield_paths installed, try disabling it for the YouTube field only.

Update: This is resolved in the latest version of filefield_paths. See: https://www.drupal.org/node/2576547