wp_post_mime_type_where does not properly prefix table names

[benbalter]

When called from the iframe media gallery, for example, wp_post_mime_type_where does not properly prefix the table name of post_mime_type in the WHERE clause, leading to a possible ambiguity if a plugin filters the media list by adding a join.

Related: #19779 and #12750.

Steps to reproduce:

1. Add a filter to posts_where that joins wp_posts (e.g., attachment:example-filter.php:ticket:19779​)
2. Navigate to a post and attempt to insert an existing image from the gallery via the media upload iframe
3. Receive an error message that post_mime_type is ambiguous

I propose that when no table alias is given, the function defaults to $wpdb->posts to prevent ambiguity.

Example resulting (erroneous) query: ​https://github.com/benbalter/WP-Document-Revisions/issues/3

Looks like this came in with [14478]. Rather than changing how the function works, we should probably just always pass $wpdb->posts in query.php.

In [20325]:

Always pass a table alias to wp_post_mime_type_where(). Prevents ambiguous queries when adding joins later on. props benbalter, fixes #20193.