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To have better suggestions for your MIME types while attaching a file
to a mail you should setup a %%PREFIX%%/etc/mime.types file.
Alternatively/additionally your users can setup a mime.types file in
their ${HOME}/.sylpheed/ directory.

If you do not know how to do that or if you do not want to invest your 
time in writting one: a good mime.types file is included with the
Apache webserver.
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