Comment 1 for bug 1761528

mime_is_default_digest only controls the digest format setting for new users. Each user has her own setting which is the 'plain' checkbox on the admin Membership Management... -> Membership Listpage or the "Get MIME or Plain Text Digests?" setting on the user's options page.

In order to change the format of digests YOU recieve, you must set YOUR option. As I said, mime_is_default_digest only controls the default digest format setting for new users.