Mark import attachments as private. Schedule job to delete old import attachments. Introduce attachment context.

git-svn-id: https://develop.svn.wordpress.org/trunk@17999 602fd350-edb4-49c9-b593-d223f7449a82

wp-admin/custom-header.php

@@ -644,7 +644,8 @@ wp_nonce_field( 'custom-header-options', '_wpnonce-custom-header-options' ); ?>
 		'post_title' => $filename,
 		'post_content' => $url,
 		'post_mime_type' => $type,
-		'guid' => $url);
+		'guid' => $url,
+		'context' => 'custom-header');
 
 		// Save the data
 		$id = wp_insert_attachment($object, $file);
@@ -737,7 +738,8 @@ wp_nonce_field( 'custom-header-options', '_wpnonce-custom-header-options' ); ?>
 			'post_title' => basename($cropped),
 			'post_content' => $url,
 			'post_mime_type' => 'image/jpeg',
-			'guid' => $url
+			'guid' => $url,
+			'context' => 'custom-header'
 		);
 
 		// Update the attachment