Unsafe regex pattern in mimemail_extract_files

The mimemail_extract_files function regex $pattern matches more items than it should. In our case, it matched the "src" suffix of a query parameter on a link.

Example: <a href="http://www.example.com?extsrc=foo"> was transformed into <a href="http://www.example.com?extsrc=Array">

because the "src=[\'"]?" clause of the regex matched.