Multi-byte characters in multipart/form-data get corrupted in RESTEasy

Issue

• When sending a text attachment that contains multi-byte characters (for example, Scandic characters like Euro €), we confirmed that it actually reaches the RESTEasy service. However, the text attachment is corrupted when saving it as a file.
• The following is the source code:

@Path("/multipart")
public class MultipartService {
    @POST
    @Consumes(MediaType.MULTIPART_FORM_DATA)
    public Response processAttachments(MultipartInput input, @Context ServletContext context) throws Exception {
        List<InputPart> parts = input.getParts ();
        for (InputPart part : parts) {
            byte[] bytes = part.getBody(byte[].class, null);
            String writeFileLocation = context.getInitParameter("write.file.location");
            FileOutputStream fileOutputStream = new FileOutputStream(new File(writeFileLocation));
            IOUtils.write(bytes, fileOutputStream);
            IOUtils.closeQuietly(fileOutputStream);
        }
        return Response.ok().build();
    }