For ppt files it must work, check your mime.types nginx config:
./mime.types: application/vnd.ms-powerpoint ppt;
For another formats you must add something similar
2013/12/6 Lorenzo Raffio <multiformeingegno@gmail.com>:
> Nginx 1.5.7, ppt(x) and pps(x) are served as plain text (of course with
> strange characters) by nginx.
>
> How can I have them always as a download?
>
>
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> http://mailman.nginx.org/mailman/listinfo/nginx
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С уважением, Соколов Евгений
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