posts: optimize filetype: searches.

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.

app/models/post.rb

@@ -1075,7 +1075,7 @@ class Post < ApplicationRecord
         when *AutocompleteService::POST_STATUSES
           status_matches(value, current_user)
         when *MediaAsset::FILE_TYPES
-          attribute_matches(value, :file_ext, :enum)
+          attribute_matches(value, "media_assets.file_ext", :enum).joins(:media_asset)
         when *Post::RATINGS.values.map(&:downcase)
           rating_matches(value)
         when *Post::RATING_ALIASES.keys