RESOLVED: I used apache library to send multipart request. It produced header:
Content-Type: multipart/form-data; boundary=tYhL_Uv6shSE9A9DHwvtJvXb0NPSLNMsdg; charset=UTF-8
When I remove encoding settings, it changed to:
Content-Type: multipart/form-data; boundary=tYhL_Uv6shSE9A9DHwvtJvXb0NPSLNMsdg
And now all works smooth.