Revert "Don't set unknown mimeTypes to "application/octet-stream""

This reverts commit f64d0879264f7b681c8d27063d35c0df36cdab47.
Revert "When mimeType is not supplied, try to fetch it from server and redo checks for automatic snapshots."

This reverts commit 6dd94f0a3ae2be60b0c6ebd4c27559f679adaaad.
Don't break in IE