The enum already has `allowedMimeTypes()` — it's the single source of
truth for what each media type accepts (jpeg/png/gif/webp for image,
mp4/quicktime/webm for video). MediaAttacher was duplicating the same
arrays as ALLOWED_IMAGE_MIMES / ALLOWED_VIDEO_MIMES constants, which
meant any change to the supported MIMEs needed two edits.
Drop the constants. `resolveType()` now walks
`[MediaType::Image, MediaType::Video]` and asks each type whether the
MIME is on its allow-list. Document type is intentionally excluded so
PDFs aren't accepted via URL fetch.
Note on size: kept MediaAttacher's own MAX_BYTES = 50 MB (separate from
the enum's per-type caps). URL fetches have different operational
constraints than direct uploads (bandwidth, timeout, unbounded user
input) — a smaller cap here is intentional.
Out of scope but worth a follow-up: `StoreAssetRequest` and
`storeChunked` are also drifted (web only accepts video/mp4 today,
the enum + MediaAttacher accept mov/webm too). They should also read
from the enum eventually.
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