使用RestClient无法检测到响应的内容类型

Question

我的Grails应用程序连接到另一个Grails应用程序，如果.pdf是单个页面文件，则将.pdf文件转换为.png；如果.pdf文件包含多个页面，则将其转换为.png的.zip。这是转换器的代码：

class PdfToPngController {
    def converterService

    def convert() {
        File originalFile = ...
        File converted = converterService.convert(originalFile, "png")

        InputStream inputStream = converted.newInputStream()

        response.outputStream << inputStream
        response.setContentLength(Long.valueOf(converted.length()).intValue())
        response.setCharacterEncoding("UTF-8")

        String mimeType
        if(converted.endsWith(".png")) {
            mimeType = "image/png"
        }
        else if(converted.endsWith(".zip")) {
            mimeType = "application/zip"
        }

        response.contentType = mimeType
        response.setHeader("Content-type", mimeType)
        response.setContentType(mimeType)

        response.setHeader("Content-disposition", "inline;filename=${converted.getName()}")

        inputStream.close()
    }
}

我的应用程序将发送pdf文件，并等待转换后的文件作为响应，问题是当我尝试获取响应的内容类型时，它返回null。这是我的应用程序上的RESTClient：

RESTClient client = new RESTClient(grailsApplication.config.report.converter.base)

client.request(Method.POST) { multipartRequest ->
    uri.path = grailsApplication.config.report.converter.path

    // ...    

    response.success = { response, data ->
        if(response.getStatus() == HttpServletResponse.SC_OK) {
            println "getContentType: ${response.getEntity().getContentType()}"
            // This prints
            // getContentType: null

            String extension
            if(response.getEntity().getContentType() == "image/png")) {
                extension = "png"
            }
            else if(response.getEntity().getContentType() == "application/zip")) {
                extension = "zip"
            }

            OutputStream outputStream = new ByteArrayOutputStream()
            response.entity.writeTo(outputStream)

            byte[] byteArray = outputStream.toByteArray()

            digest = md5.digest(byteArray)
            hashedFilename = new BigInteger(1, digest).toString(36)

            FileUtils.writeByteArrayToFile(new File(thumbnailsDir.getAbsolutePath() +
                File.separator + hashedFilename +
                "." + extension), byteArray)

            outputStream.close()
        }
        else if(HttpServletResponse.SC_CREATED) {
            throw new ConnectException("The report service failed to convert it to .png.")
        }
    }
}

为什么我的应用程序无法确定响应的contentType？转换器应用程序是否将contentType设置为错误？还是在确定contentType时使用了错误的方法？还是转换应用程序的错误？