Index: wp-includes/class-wp-image-editor.php
===================================================================
--- wp-includes/class-wp-image-editor.php	(revision 23182)
+++ wp-includes/class-wp-image-editor.php	(working copy)
@@ -371,7 +371,8 @@
 
 		foreach( $extensions as $_extension ) {
 			if ( preg_match( "/{$extension}/i", $_extension ) ) {
-				return $mime_types[$_extension];
+				// asumes that the first mime type in the array is the most common/proper
+				return !is_array($mime_types[$_extension]) ? $mime_types[$_extension] : $mime_types[$_extension][0];
 			}
 		}
 
@@ -389,7 +390,9 @@
 	 * @return string|boolean
 	 */
 	protected static function get_extension( $mime_type = null ) {
-		$extensions = explode( '|', array_search( $mime_type, wp_get_mime_types() ) );
+		foreach ( wp_get_mime_types() as $extension => $mimes )
+			if ( $match = array_search( $mime_type, (array) $mimes ) )
+				$extensions = explode( '|', $extension );
 
 		if ( empty( $extensions[0] ) )
 			return false;