If you look at the raw post data…
while (have_posts() ) {
the_post(); ?>
<article id="post-<?php the_ID(); ?>" <?php post_class(); ?>><?php
var_dump($post); ?>
</article><?php
}
You will see that there is nothing in there to specify “medium”, so I went looking. There is a filter on the_content…
add_filter( 'the_content', 'prepend_attachment' );
… that juggles data for “attachment” post types.
1576 function prepend_attachment($content) {
1577 $post = get_post();
1578
1579 if ( empty($post->post_type) || $post->post_type != 'attachment' )
1580 return $content;
1581
1582 if ( wp_attachment_is( 'video', $post ) ) {
1583 $meta = wp_get_attachment_metadata( get_the_ID() );
1584 $atts = array( 'src' => wp_get_attachment_url() );
1585 if ( ! empty( $meta['width'] ) && ! empty( $meta['height'] ) ) {
1586 $atts['width'] = (int) $meta['width'];
1587 $atts['height'] = (int) $meta['height'];
1588 }
1589 if ( has_post_thumbnail() ) {
1590 $atts['poster'] = wp_get_attachment_url( get_post_thumbnail_id() );
1591 }
1592 $p = wp_video_shortcode( $atts );
1593 } elseif ( wp_attachment_is( 'audio', $post ) ) {
1594 $p = wp_audio_shortcode( array( 'src' => wp_get_attachment_url() ) );
1595 } else {
1596 $p = '<p class="attachment">';
1597 // show the medium sized image representation of the attachment if available, and link to the raw file
1598 $p .= wp_get_attachment_link(0, 'medium', false);
1599 $p .= '</p>';
1600 }
1601
1602 /**
1603 * Filter the attachment markup to be prepended to the post content.
1604 *
1605 * @since 2.0.0
1606 *
1607 * @see prepend_attachment()
1608 *
1609 * @param string $p The attachment HTML output.
1610 */
1611 $p = apply_filters( 'prepend_attachment', $p );
1612
1613 return "$p\n$content";
1614 }
The relevant lines are 1597 and 1598. wp_get_attachment_link(), which in turn uses wp_get_attachment_image() (line 1545), which uses image_downsize()… and there is a promising filter:
167 if ( $out = apply_filters( 'image_downsize', false, $id, $size ) ) {
168 return $out;
169 }
Et viola!
function image_size_hack() {
$img_url = wp_get_attachment_url();
$meta = wp_get_attachment_metadata();
$width= $height="";
if (!empty($meta['width'])) {
$width = $meta['width'];
}
if (!empty($meta['height'])) {
$width = $meta['height'];
}
return array( $img_url, $width, $height, false );
}
add_filter( 'image_downsize', 'image_size_hack');