The current implementation only supports single HTTP accept headers with comma separated values. Following the RFC 2616 (http://www.w3.org/Protocols/rfc2616/rfc2616-sec4.html#sec4.2 ) multiple (accept) headers are allowed as well.
This behaviour is implemented for example by OpenRDF Alibaba:
POST /marmotta/sparql/select HTTP/1.1
Content-Type: application/x-www-form-urlencoded; charset=utf-8
The current implementation returns a HTTP 415 (Unsupported Media Type
) because only the format application/x-binary-rdf-results-table is considered.