[Cocoa] Files from webmfiles.org do not play through MSE
<rdar://74640660>
Created attachment 421501 [details] Patch
Created attachment 421503 [details] Patch
Created attachment 421541 [details] Patch for landing
Committed r273501: <https://commits.webkit.org/r273501> All reviewed patches have been landed. Closing bug and clearing flags on attachment 421541 [details].