FileUpload.scala

http x((:/ ("www.mysite.com") / "upload") <<
    ("file", new java.io.File("""path\to\file\""")) as_str)
}

InputStreamUpload.scala

http x((:/ ("www.mysite.com") / "upload") <<
    ("file", "myFilename", () => inputStream) as_str)
}

SuperMimeRequest.scala

import dispatch._
import dispatch.Http._
import dispatch.mime.Mime._

class FileBodyFromStream(
  filename: String, 
  inputStream: () => java.io.InputStream, 
  lengthOpt: Option[Long] = None
) extends org.apache.http.entity.mime.content.FileBody(new java.io.File("")) {
  val (writeToFunc, length) = lengthOpt.map(length=>
    ((out: java.io.OutputStream) => Stream.continually(inputStream().read).
        takeWhile(_ != -1).foreach(out.write(_)), 
      length
    )
  ) getOrElse { // No length supplied, read stream to get length
    val buffer = new java.io.ByteArrayOutputStream()
    Stream.continually(inputStream().read).takeWhile(_ != -1).foreach(buffer.write(_))
    ((out: java.io.OutputStream) => buffer.writeTo(out), 
      buffer.size().toLong
    )
  }
  override def writeTo(out: java.io.OutputStream) = writeToFunc(out)
  override def getContentLength() = length
  override def getFilename() = filename
}

class SuperMimeRequest(r: Request) extends MimeRequest(r) {
  def <<* (
    name: String, 
    filename: String, 
    inputStream: ()=>java.io.InputStream, 
    length: Option[Long] = None
  ) = r next add(name, new FileBodyFromStream(filename, inputStream, length))	
}

object SyncApp extends Application {
  implicit def Request2SuperMimeRequest(r: Request) = new SuperMimeRequest(r)
  val (filename, inputStream, contentLength) = ...
  http x((:/ ("www.mysite.com") / "upload") <<* 
    ("file", filename, () => inputStream, Some(contentLength)) as_str)
}