edited code

public function imageAction()
    {
    $id = $this->_getParam('id');

    $dokumentTabell = new Ordre_Model_Table_OrdreDokument();
    $dokument = $dokumentTabell->find($id)->current();
    $this->view->dokument = $dokument;
    $filename = $dokument->Name.".".$dokument->FileExtension;
    // disable layout and view
    $this->view->layout()->disableLayout();
    $this->_helper->viewRenderer->setNoRender(true);

    $this->getResponse()
         ->clearAllHeaders()
         ->clearBody()
         ->setHttpResponseCode(200)
         ->setHeader('Content-Type', 'image/jpeg')
         ->setHeader('Content-Length', strlen($dokument->Dokument))
         ->setHeader(
            'Content-Disposition',
            "attachment;filename=\"{$filename}\""
         )
         ->setHeader('Last-Modified', $dokument->SistEndret)
         ->setBody($dokument->Dokument);
    }