Unable to get filename in binary multipart/form-data

Steps to reproduce the problem:

1. mitmproxy -p 8888
2. curl -X POST "http://google.com/upload" -H 'Content-type:multipart/form-data; boundary=----------a8ba1f6ad84944199d5346f69bb14140' --data-binary $'------------a8ba1f6ad84944199d5346f69bb14140/\r\nContent-Disposition: form-data; name="file"; filename="error.log.zip";\r\nContent-Type: application/octet-stream\r\n\r\nx\x9c\xbd\x96\xddR\xab0\x10\xc7\xaf\xedS\xe4\xc2\r\n------------a8ba1f6ad84944199d5346f69bb14140--\r\n' -x localhost:8888
3. Check request data, only "file" field without filename

System information

Mitmproxy: 4.0.3
Python:    3.6.5
OpenSSL:   OpenSSL 1.1.0h  27 Mar 2018
Platform:  Linux-4.9.4-galliumos-braswell-x86_64-with-Ubuntu-16.04-xenial

Reproduce the request with the provided curl command through mitmproxy on port 8888, then inspect how the request data represents the multipart field. Trace the multipart parsing entry point from the observed request data and verify that the filename from Content-Disposition is preserved. Done means the file field exposes error.log.zip while the binary payload remains intact.