In SurgeMail 7.0b-2 and later you can enable this feature to extract and append normal attachments to emails that contain a 'winmail.dat' file.
g_winmail_fix "true"
You will need to also install the tnef binary:
Unix: apt-get install tnef (Surgemail expects to find it as /usr/local/bin/tnef)
Windows: https://netwinsite.com/ftp/tnef/tnef.exe (and store in the c:\surgemail folder)
Please note: This windows binary is built from this source: https://github.com/verdammelt/tnef release 1.4.12
Alternatively, you can tell surgemail to reject winmail.dat to force the user to fix their email client settings:
g_winmail_reject "true"
Or you can reject 'outgoing' winmail.dat files sent by your local users, so at least 'your' users can fix their settings and not annoy the rest of the internet.
g_winmail_reject_send "true"