Function provider_loadable_image_mime_for 

pub fn provider_loadable_image_mime_for(
    file_name: &str,
    data: &[u8],
) -> Option<&'static str>

The provider-loadable image MIME for a (file_name, bytes) pair, or None when the loader would reject it. Free-standing so a channel can consult the same contract before it owns a MediaAttachment: Discord decides a marker’s disposition from the borrowed download bytes, and duplicating the precedence here would let the two drift. MediaAttachment::provider_loadable_image_mime is the borrowing convenience over this.