attempt at using extra attribute with Maven repo

build.sbt

lazy val root = (project in file(".")).
  settings(
    bintrayReleaseOnPublish in ThisBuild := false
  )

lazy val libExtra = (project in file("libExtra")).
  settings(
    version := "0.1",
    scalaVersion := "2.11.7",
    organization := "com.example",
    name := "somelibrary",
    licenses += ("MIT", url("http://opensource.org/licenses/MIT")),
    bintrayVcsUrl := Some("git@github.com:you/your-repo.git"),
    bintrayOrganization := None,
    bintrayRepository := "test-test-test",
    publishMavenStyle := true,
    checksums in publish := Nil,
    artifact in (Compile, packageBin) := {
      val previous: Artifact = (artifact in (Compile, packageBin)).value
      previous.extra("branch_name" -> "master-api-model-separation")
    },
    publishTo := {
      Some(URLRepository("test-bintray-maven", Patterns("https://api.bintray.com/content/eed3si9n/test-test-test/[organisation]/[module](_[branch_name])(_[scalaVersion])(_[sbtVersion])/[revision]/[artifact]-[revision](-[classifier]).[ext];bt_package=somelibrary;bt_version=0.1")))
    }
  )

lazy val app = (project in file("app")).
  settings(
    scalaVersion := "2.11.7",
    organization := "foo",
    libraryDependencies += "com.example" %% "somelibrary" % "0.1" extra("branch_name" -> "master-api-model-separation"),
    resolvers += Resolver.url("test-bintray-maven", url("https://dl.bintray.com/eed3si9n/test-test-test/"))(Patterns(true, "[organisation]/[module](_[branch_name])(_[scalaVersion])(_[sbtVersion])/[revision]/[artifact]-[revision](-[classifier]).[ext]")),
    fullResolvers := fullResolvers.value.filterNot(_.name == "inter-project")
  )