Fix index value that was left over from 1.4.x

functions/attachment_common.php

@@ -265,7 +265,7 @@ function attachment_common_octet_stream(&$Args) {
 //FIXME: or at least we can move this hook up to the top of this file where $FileExtensionToMimeType is defined.  What else is this hook here for?  What plugins use it?
     do_hook('attachment_common-load_mime_types', $null);
 
-    preg_match('/\.([^.]+)$/', $Args[7], $Regs);
+    preg_match('/\.([^.]+)$/', $Args[6], $Regs);
 
     $Ext = '';
     if (is_array($Regs) && isset($Regs[1])) {