Add DownloadableDatatypeHandler interface

interface DownloadableDatatypeHandler extends CustomDatatypeHandler {
    /**
     * Gets the mime type of downloading the given custom value.
     * Must not return null.
     */
    String getMimeType(CustomDatatype dt, String valueReference);

    /**
     * Gets the filename downloading the given custom value.
     * May return null, in which case the framework will choose a meaningless filename.
     */
    String getFilename(CustomDatatype dt, String valueReference);

    /**
     * Writes the custom value to os.
     * Implementations should assume that getMimeType and getFilename have been called and applied.
     * Implementations should assume that os is some sort of BufferedOutputStream
     */
    void writeToStream(OutputStream os);
}

// I'm not sure why we'd need to give such low-level control to implementations
interface DownloadableDatatypeHandler extends CustomDatatypeHandler {
    /**
     *
     */
    void writeResponse(HttpServletResponse);
}

abstract class BaseDownloadableDatatypeHandler implements CustomDatatypeHandler {
    abstract String getMimeType(CustomDatatype dt, String valueReference);
    abstract String getFilename(CustomDatatype dt, String valueReference);
    abstract void writeToStream(OutputStream os);
}