mhbuild: Transfer-encode MIME parts of type application always with base64

Application files likely contain NUL bytes. scan_content(), however, uses
stdio to check the files to attach. NUL bytes break these checks. Instead
of improving the checks to allow e.g. application/x-tex files to be sent
using 7bit or QP encoding, it was chosen to take the easy way and send
application files base64-encoded.

This solves a bug when attaching tar files. (tar files begin with a
header mostly filled with NUL bytes). Thanks to <philipp@bureaucracy.de>
for discovering and reporting the bug.