file read error when files folder is mounted

Issue Summary

Hello,

In general, we're mounting s3 for our Drupal files folder. So the layout is something like:

/var/files (s3fs mount)
/var/www/sites/default/files (symbolic link to /var/files)

Everything is working well, except we found an error in mimemail where php memory is exhausted regardless of what it's set to. The fix for me was to simply append the $base_url.base_path() to it. Not sure if this bug applies to all mounts or just s3fs, I think the better place to report a bug is either in s3fs or in php's file_get_contents. In the meantime, this patch solves the problem. Though I'm not sure it's the best way to solve it.