### What?
This PR relaxes the mimeType checks in the thumbnail and file cell
components to accommodate an `adminThumbnail` even if the file is a
non-image. This is useful when, for example, using an `adminThumbnail`
function to retrieve or generate thumbnails for files that are
non-images such as videos.
### Why?
To prioritize an admin thumbnail if/when available on file cells and
upload field thumbnails in both edit and list views.
### How?
By relaxing the mimeType checks in the `Thumbnail` component and instead
lifting that responsibility on the caller of this component. Some of
these checks were not needed as the best-fit helper utility function
will automatically select the thumbnailURL if available or revert to the
original url if no best-fit is found.
Demo of admin thumbnail being loaded on non-image while still selecting
best-fit size for images:
77210251f4