file_view_multiple() now returns all results inside $return['files']

This was to make the file_view_multiple() function compatible with Entity API's entity_view() function.

Before:

$output = file_view_multiple($files, 'full');
// Return the file 10's output.
return $output[10];

After:

$output = file_view_multiple($files, 'full');
// Return the file 10's output.
return $output['files'][10];