Zip files which do not contain a bare parent directory entry were not being detected correctly.

easy-theme-and-plugin-upgrades.diff

Index: modify-installer.php
===================================================================
--- modify-installer.php	(revision 826618)
+++ modify-installer.php	(working copy)
@@ -134,7 +134,7 @@
 			$contents = $archive->listContent();
 			
 			foreach ( (array) $contents as $content ) {
-				if ( preg_match( '|^([^/]+)/$|', $content['filename'], $matches ) ) {
+				if ( preg_match( '|^([^/]+)/|', $content['filename'], $matches ) ) {
 					$directory = $matches[1];
 					break;
 				}