Change the Accept header based on a query parameter in Play. (http://mysite/stuff/1?as=json)

PimpAcceptHeader.scala

import play.api.http.{ HeaderNames, MimeTypes }
import play.api.mvc.{ Headers, Handler, RequestHeader }
import scala.collection.mutable.ArrayBuffer
import play.api.GlobalSettings

object Global extends GlobalSettings {

  override def onRequestReceived(request: RequestHeader): (RequestHeader, Handler) = {
    val defaultHeader = super.onRequestReceived(request)
    val (header, handler) = defaultHeader
    if (header.path.startsWith("/assets/")) defaultHeader
    else {
      val newRequest = for {
        asValue <- header.getQueryString("as")
        mimeType <- mapMimeType(asValue)
        requestHeader <- overrideAcceptHeader(mimeType, request)
      } yield (requestHeader, handler)
      newRequest.getOrElse(defaultHeader)
    }
  }

  def mapMimeType(value: String) = value match {
    case "json" => Some(MimeTypes.JSON)
    case "html" => Some(MimeTypes.HTML)
    case "mol" => Some("chemical/x-mdl-molfile")
    case _ => None
  }

  def overrideAcceptHeader(mineType: String, requestHeader: RequestHeader) =
    requestHeader.headers.get(HeaderNames.ACCEPT)
      .map(header => newReq(requestHeader, mineType + "," + header))

  def newReq(header: RequestHeader, newAccept: String) = new RequestHeader {
    def uri = header.uri
    def remoteAddress = header.remoteAddress
    def queryString = header.queryString
    def method = header.method
    def headers: Headers = new Headers {
      protected val data =
        header.headers.toMap
          .updated(HeaderNames.ACCEPT, ArrayBuffer(newAccept)).toSeq
    }
    def path = header.path
    def version = header.version
    def tags = header.tags
    def id: Long = header.id
  }
}