Source

File: wp-includes/post.php

function wp_mime_type_icon( $mime = 0 ) {
	if ( ! is_numeric( $mime ) ) {
		$icon = wp_cache_get( "mime_type_icon_$mime" );
	}

	$post_id = 0;
	if ( empty( $icon ) ) {
		$post_mimes = array();
		if ( is_numeric( $mime ) ) {
			$mime = (int) $mime;
			$post = get_post( $mime );
			if ( $post ) {
				$post_id = (int) $post->ID;
				$file    = get_attached_file( $post_id );
				$ext     = preg_replace( '/^.+?\.([^.]+)$/', '$1', $file );
				if ( ! empty( $ext ) ) {
					$post_mimes[] = $ext;
					$ext_type     = wp_ext2type( $ext );
					if ( $ext_type ) {
						$post_mimes[] = $ext_type;
					}
				}
				$mime = $post->post_mime_type;
			} else {
				$mime = 0;
			}
		} else {
			$post_mimes[] = $mime;
		}

		$icon_files = wp_cache_get( 'icon_files' );

		if ( ! is_array( $icon_files ) ) {
			/**
			 * Filters the icon directory path.
			 *
			 * @since 2.0.0
			 *
			 * @param string $path Icon directory absolute path.
			 */
			$icon_dir = apply_filters( 'icon_dir', ABSPATH . WPINC . '/images/media' );

			/**
			 * Filters the icon directory URI.
			 *
			 * @since 2.0.0
			 *
			 * @param string $uri Icon directory URI.
			 */
			$icon_dir_uri = apply_filters( 'icon_dir_uri', includes_url( 'images/media' ) );

			/**
			 * Filters the array of icon directory URIs.
			 *
			 * @since 2.5.0
			 *
			 * @param string[] $uris Array of icon directory URIs keyed by directory absolute path.
			 */
			$dirs       = apply_filters( 'icon_dirs', array( $icon_dir => $icon_dir_uri ) );
			$icon_files = array();
			while ( $dirs ) {
				$keys = array_keys( $dirs );
				$dir  = array_shift( $keys );
				$uri  = array_shift( $dirs );
				$dh   = opendir( $dir );
				if ( $dh ) {
					while ( false !== $file = readdir( $dh ) ) {
						$file = wp_basename( $file );
						if ( '.' === substr( $file, 0, 1 ) ) {
							continue;
						}

						$ext = strtolower( substr( $file, -4 ) );
						if ( ! in_array( $ext, array( '.png', '.gif', '.jpg' ), true ) ) {
							if ( is_dir( "$dir/$file" ) ) {
								$dirs[ "$dir/$file" ] = "$uri/$file";
							}
							continue;
						}
						$icon_files[ "$dir/$file" ] = "$uri/$file";
					}
					closedir( $dh );
				}
			}
			wp_cache_add( 'icon_files', $icon_files, 'default', 600 );
		}

		$types = array();
		// Icon wp_basename - extension = MIME wildcard.
		foreach ( $icon_files as $file => $uri ) {
			$types[ preg_replace( '/^([^.]*).*$/', '$1', wp_basename( $file ) ) ] =& $icon_files[ $file ];
		}

		if ( ! empty( $mime ) ) {
			$post_mimes[] = substr( $mime, 0, strpos( $mime, '/' ) );
			$post_mimes[] = substr( $mime, strpos( $mime, '/' ) + 1 );
			$post_mimes[] = str_replace( '/', '_', $mime );
		}

		$matches            = wp_match_mime_types( array_keys( $types ), $post_mimes );
		$matches['default'] = array( 'default' );

		foreach ( $matches as $match => $wilds ) {
			foreach ( $wilds as $wild ) {
				if ( ! isset( $types[ $wild ] ) ) {
					continue;
				}

				$icon = $types[ $wild ];
				if ( ! is_numeric( $mime ) ) {
					wp_cache_add( "mime_type_icon_$mime", $icon );
				}
				break 2;
			}
		}
	}

	/**
	 * Filters the mime type icon.
	 *
	 * @since 2.1.0
	 *
	 * @param string $icon    Path to the mime type icon.
	 * @param string $mime    Mime type.
	 * @param int    $post_id Attachment ID. Will equal 0 if the function passed
	 *                        the mime type.
	 */
	return apply_filters( 'wp_mime_type_icon', $icon, $mime, $post_id );
}