The problem is that it's indeed cp1252 (yes, I hate retarded people too) character in the body.
curl -kx127.0.0.8:8080 'http://httpbin.org/post' -H 'Content-Type: multipart/form-data; boundary=a' --data-binary $'--a\r\nContent-Disposition: form-data; name="ctl00$BodyContent$txtAtesia"\r\n\r\nDuk\xeb\r\n--a--\r\n'
I've tried request.content and request.raw_content both, mitmproxy crashes. SO doesn't want me to post traceback, but it's reproducible.
Nevermind, request.text works.
Should mitmproxy developers happen to pass by, they can deal with the crash though:)