Index: wp-includes/class-wp-xmlrpc-server.php
===================================================================
--- wp-includes/class-wp-xmlrpc-server.php	(revision 19641)
+++ wp-includes/class-wp-xmlrpc-server.php	(working copy)
@@ -3085,12 +3085,24 @@
 			logIO('O', '(MW) ' . $errorString);
 			return new IXR_Error(500, $errorString);
 		}
+
+		$caption = '';
+		if ( isset( $data['caption'] ) ) {
+			$caption = $data['caption'];
+		}
+
+		$description = '';
+		if ( isset( $data['description'] ) ) {
+			$description = $data['description'];
+		}
+
 		// Construct the attachment array
 		// attach to post_id 0
 		$post_id = 0;
 		$attachment = array(
 			'post_title' => $name,
-			'post_content' => '',
+			'post_excerpt' => $caption,
+			'post_content' => $description,
 			'post_type' => 'attachment',
 			'post_parent' => $post_id,
 			'post_mime_type' => $type,