For recent versions of Nexus OSS (>= 3.9.0)
Example for versions 3.9.0 to 3.13.0:
curl -D - -u user:pass -X POST "https://nexus.domain/nexus/service/rest/beta/components?repository=somerepo" -H "accept: application/json" -H "Content-Type: multipart/form-data" -F "raw.directory=/test/" -F "[email protected];type=application/json" -F "raw.asset1.filename=test.txt"