Java – Cannot consume content type – JAX-RS

If a client connects to a web service (JAX-RS) like this:

URL u = new URL(server);
URLConnection con = u.openConnection();
con.setDoOutput(true);
con.getOutputStream().write(stream.toByteArray()); // ByteArrayOutputStream 
con.connect();
InputStream inputStream = con.getInputStream();
byte [] urlBytes = new byte [inputStream.available()];
inputStream.read(urlBytes);
url = new String(urlBytes);

Then what should be the web service interface definition? I have:

@POST 
@Path("/upload")
@Consumes("image/jpeg")
@Produces(MediaType.TEXT_PLAIN)
String uploadPicture();

However, when accessed by the client, it throws:

[ERROR] INFO: generating jax-rs proxy loader class. [INFO] DEBUG
[SynchronousDispatcher] PathInfo: /blob/upload [INFO] WARN
[ExceptionHandler] Failed executing POST /blob/upload [INFO]
org.jboss.resteasy.spi.UnsupportedMediaTypeException: Cannot consume
content type [INFO] at
org.jboss.resteasy.core.registry.Segment.match(Segment.java:117)
[INFO] at
org.jboss.resteasy.core.registry.SimpleSegment.matchSimple(SimpleSegment.java:33)
[INFO] at
org.jboss.resteasy.core.registry.RootSegment.matchChildren(RootSegment.java:327)
[INFO] at
org.jboss.resteasy.core.registry.SimpleSegment.matchSimple(SimpleSegment.java:44)
[INFO] at
org.jboss.resteasy.core.registry.RootSegment.matchChildren(RootSegment.java:327)
[INFO] at
org.jboss.resteasy.core.registry.RootSegment.matchRoot(RootSegment.java:374)
[INFO] at
org.jboss.resteasy.core.registry.RootSegment.matchRoot(RootSegment.java:367)
[INFO] at
org.jboss.resteasy.core.ResourceMethodRegistry.getResourceInvoker(ResourceMethodRegistry.java:354)
[INFO] at
org.jboss.resteasy.core.SynchronousDispatcher.getInvoker(SynchronousDispatcher.java:231)
[INFO] at
org.jboss.resteasy.core.SynchronousDispatcher.invoke(SynchronousDispatcher.java:170)
[INFO] at
org.jboss.resteasy.plugins.server.servlet.ServletContainerDispatcher.service(ServletContainerDispatcher.java:208)
[INFO] at
org.jboss.resteasy.plugins.server.servlet.HttpServletDispatcher.service(HttpServletDispatcher.java:56)
[INFO] at
org.jboss.resteasy.plugins.server.servlet.HttpServletDispatcher.service(HttpServletDispatcher.java:51)
[INFO] at
javax.servlet.http.HttpServlet.service(HttpServlet.java:717) [INFO]
at
org.mortbay.jetty.servlet.ServletHolder.handle(ServletHolder.java:511)
[INFO] at
org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1166)
[INFO] at
com.google.inject.servlet.FilterChainInvocation.doFilter(FilterChainInvocation.java:66)
[INFO] at
com.googlecode.objectify.cache.AsyncCacheFilter.doFilter(AsyncCacheFilter.java:39)
[INFO] at
com.google.inject.servlet.FilterDefinition.doFilter(FilterDefinition.java:163)
[INFO] at
com.google.inject.servlet.FilterChainInvocation.doFilter(FilterChainInvocation.java:58)
[INFO] at
org.apache.shiro.web.servlet.AbstractShiroFilter.executeChain(AbstractShiroFilter.java:449)
[INFO] at
org.apache.shiro.web.servlet.AbstractShiroFilter$1.call(AbstractShiroFilter.java:365)
[INFO] at
org.apache.shiro.subject.support.SubjectCallable.doCall(SubjectCallable.java:90)
[INFO] at
org.apache.shiro.subject.support.SubjectCallable.call(SubjectCallable.java:83)
[INFO] at
org.apache.shiro.subject.support.DelegatingSubject.execute(DelegatingSubject.java:383)
[INFO] at
org.apache.shiro.web.servlet.AbstractShiroFilter.doFilterInternal(AbstractShiroFilter.java:362)
[INFO] at
org.apache.shiro.web.servlet.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:125)
[INFO] at
com.google.inject.servlet.FilterDefinition.doFilter(FilterDefinition.java:163)
[INFO] at
com.google.inject.servlet.FilterChainInvocation.doFilter(FilterChainInvocation.java:58)
[INFO] at
com.google.inject.servlet.ManagedFilterPipeline.dispatch(ManagedFilterPipeline.java:118)
[INFO] at
com.google.inject.servlet.GuiceFilter.doFilter(GuiceFilter.java:113)
[INFO] at
org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
[INFO] at
com.google.appengine.api.socket.dev.DevSocketFilter.doFilter(DevSocketFilter.java:74)
[INFO] at
org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
[INFO] at
com.google.appengine.tools.development.ResponseRewriterFilter.doFilter(ResponseRewriterFilter.java:123)
[INFO] at
org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
[INFO] at
com.google.appengine.tools.development.HeaderVerificationFilter.doFilter(HeaderVerificationFilter.java:34)
[INFO] at
org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
[INFO] at
com.google.appengine.api.blobstore.dev.ServeBlobFilter.doFilter(ServeBlobFilter.java:63)
[INFO] at
org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
[INFO] at
com.google.apphosting.utils.servlet.TransactionCleanupFilter.doFilter(TransactionCleanupFilter.java:43)
[INFO] at
org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
[INFO] at
com.google.appengine.tools.development.StaticFileFilter.doFilter(StaticFileFilter.java:125)
[INFO] at
org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
[INFO] at
com.google.appengine.tools.development.DevAppServerServersFilter.doDirectRequest(DevAppServerServersFilter.java:369)
[INFO] at
com.google.appengine.tools.development.DevAppServerServersFilter.doDirectServerRequest(DevAppServerServersFilter.java:352)
[INFO] at
com.google.appengine.tools.development.DevAppServerServersFilter.doFilter(DevAppServerServersFilter.java:115)
[INFO] at
org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
[INFO] at
org.mortbay.jetty.servlet.ServletHandler.handle(ServletHandler.java:388)
[INFO] at
org.mortbay.jetty.security.SecurityHandler.handle(SecurityHandler.java:216)
[INFO] at
org.mortbay.jetty.servlet.SessionHandler.handle(SessionHandler.java:182)
[INFO] at
org.mortbay.jetty.handler.ContextHandler.handle(ContextHandler.java:765)
[INFO] at
org.mortbay.jetty.webapp.WebAppContext.handle(WebAppContext.java:418)
[INFO] at
com.google.appengine.tools.development.DevAppEngineWebAppContext.handle(DevAppEngineWebAppContext.java:97)
[INFO] at
org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:152)
[INFO] at
com.google.appengine.tools.development.JettyContainerService$ApiProxyHandler.handle(JettyContainerService.java:438)
[INFO] at
org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:152)
[INFO] at org.mortbay.jetty.Server.handle(Server.java:326) [INFO] at
org.mortbay.jetty.HttpConnection.handleRequest(HttpConnection.java:542)
[INFO] at
org.mortbay.jetty.HttpConnection$RequestHandler.content(HttpConnection.java:938)
[INFO] at org.mortbay.jetty.HttpParser.parseNext(HttpParser.java:755)
[INFO] at
org.mortbay.jetty.HttpParser.parseAvailable(HttpParser.java:212)
[INFO] at
org.mortbay.jetty.HttpConnection.handle(HttpConnection.java:404)
[INFO] at
org.mortbay.io.nio.SelectChannelEndPoint.run(SelectChannelEndPoint.java:409)
[INFO] at
org.mortbay.thread.QueuedThreadPool$PoolThread.run(QueuedThreadPool.java:582)

What should be the JAX-RS interface for this client?

Update:

One thing to note here is that the client code is already compiled and signed and I can't just change it.