Issue 1:
I have searched the issues of this repository and believe that this is not a duplicate.

I have checked the FAQ of this repository and believe that this is not a duplicate.

Environment

Dubbo version: 2.6.4

Operating System version: linux

Java version: jdk1.8

Steps to reproduce this issue

provider提供者自定义loadbalance扩展

<dubbo:service interface="com.xxx.XxApi" ref="xxApi">

<dubbo:method name="method" loadbalance="hashLoadBalance">

<dubbo:parameter key="hash.arguments" value="0,1" />

</dubbo:method>

</dubbo:service>

服务提供者文件配置对应扩展

src

|-main

|-java

|-com

|-xxx

|-XxxLoadBalance.java (实现LoadBalance接口)

|-resources

|-META-INF

|-dubbo

|-com.alibaba.dubbo.rpc.cluster.LoadBalance (纯文本文件，内容为：hashLoadBalance=com.xxx.HashLoadBalance)

3.消费者调用

` @reference

private XxApi xxApi;

public void xx() {

xxApi.method()

}

`

Pls. provide [GitHub address] to reproduce this issue.

Actual Result

消费者调用不能识别提供者方法上配置的自定义hashLoadBalance扩展

java.lang.IllegalStateException: No such extension com.alibaba.dubbo.rpc.cluster.LoadBalance by name hashLoadBalance
at com.alibaba.dubbo.common.extension.ExtensionLoader.findException(ExtensionLoader.java:482)
at com.alibaba.dubbo.common.extension.ExtensionLoader.createExtension(ExtensionLoader.java:489)
at com.alibaba.dubbo.common.extension.ExtensionLoader.getExtension(ExtensionLoader.java:309)
at com.alibaba.dubbo.rpc.cluster.support.AbstractClusterInvoker.invoke(AbstractClusterInvoker.java:240)
at com.alibaba.dubbo.rpc.cluster.support.wrapper.MockClusterInvoker.invoke(MockClusterInvoker.java:75)
at com.alibaba.dubbo.rpc.proxy.InvokerInvocationHandler.invoke(InvokerInvocationHandler.java:52)
at com.alibaba.dubbo.common.bytecode.proxy0.measure(proxy0.java)
at com.daimler.visual.LbsVssManager.measure(LbsVssManager.java:38)
at com.daimler.visual.LbsVssManager$$FastClassBySpringCGLIB$$d6590229.invoke(<generated>)
at org.springframework.cglib.proxy.MethodProxy.invoke(MethodProxy.java:204)
at org.springframework.aop.framework.CglibAopProxy$CglibMethodInvocation.invokeJoinpoint(CglibAopProxy.java:747)
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:163)
at org.springframework.dao.support.PersistenceExceptionTranslationInterceptor.invoke(PersistenceExceptionTranslationInterceptor.java:139)
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:185)
at org.springframework.aop.framework.CglibAopProxy$DynamicAdvisedInterceptor.intercept(CglibAopProxy.java:689)
at com.daimler.visual.LbsVssManager$$EnhancerBySpringCGLIB$$419b98e9.measure(<generated>)
at com.daimler.visual.dubbo.CompassApiTest.test_lbs(CompassApiTest.java:69)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
at org.springframework.test.context.junit4.statements.RunBeforeTestExecutionCallbacks.evaluate(RunBeforeTestExecutionCallbacks.java:73)
at org.springframework.test.context.junit4.statements.RunAfterTestExecutionCallbacks.evaluate(RunAfterTestExecutionCallbacks.java:83)
at org.springframework.test.context.junit4.statements.RunBeforeTestMethodCallbacks.evaluate(RunBeforeTestMethodCallbacks.java:75)
at org.springframework.test.context.junit4.statements.RunAfterTestMethodCallbacks.evaluate(RunAfterTestMethodCallbacks.java:86)
at org.springframework.test.context.junit4.statements.SpringRepeat.evaluate(SpringRepeat.java:84)
at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.runChild(SpringJUnit4ClassRunner.java:251)
at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.runChild(SpringJUnit4ClassRunner.java:97)
at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
at org.springframework.test.context.junit4.statements.RunBeforeTestClassCallbacks.evaluate(RunBeforeTestClassCallbacks.java:61)
at org.springframework.test.context.junit4.statements.RunAfterTestClassCallbacks.evaluate(RunAfterTestClassCallbacks.java:70)
at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.run(SpringJUnit4ClassRunner.java:190)
at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:68)
at com.intellij.rt.execution.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:47)
at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:242)
at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:70)

Issue 2:
[ X] I have searched the issues of this repository and believe that this is not a duplicate.

[X ] I have checked the FAQ of this repository and believe that this is not a duplicate.

I add one new serialization and there can't find any useage of method Serialization.getContentType() in project.

So what string should i return. And if there are some rules of this string value.