When searching posts by width, height, file size, or file extension, use the
values from the media_assets table rather than the posts table.
This makes filetype: searches faster because the file_ext is indexed on
the media assets table, but not on the posts table.
This paves the way for getting rid of the width, height, file_size, and
file_ext indexes on the posts table in the future. It's wasteful to
index these columns on both the posts table and the media assets table.
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