HTTP MIME types question

Hello All!

I have a question about HTTP MIME types.
Our company created application which uses HTTP protocol for communications.
What is the best MIME type to use?
Can any proxy change content (e.g. for security reasons) if I use
 no HTTP MIME type?
Some of proxy servers can try to cut banners,
 remove sounds and so on. How can I avoid this?

Regards,
Voronkov Konstantin
ICQ 40202717