Extend an existing component class in JIRA

I am writing a plugin for Jira 6.3.
I want to extend the Jira interface com.atlassian.jira.bc.issue.attachment.AttachmentService

:

public interface MyAttachmentService extend AttachmentService {
    ...
}

      

        
        
        
      

    

then I implemented my interface:

public class MyDefaultAttachmentService extends DefaultAttachmentService implements MyAttachmentService {
    ...
}

      

        
        
        
      

    

I have defined my class as a component in atlassian-plugin.xml:

<component key="myAttachmentService" name="MyAttachmentService" class="com.my.plugin.issue.attachment.MyDefaultAttachmentService">
    <interface>com.my.plugin.issue.attachment.MyAttachmentService</interface>
</component>

      

        
        
        
      

    

How I use my plugin class (dependency injection):

public class DoSomethingWithAttachmentAction extends AbstractIssueSelectAction 
{
    private final MyAttachmentService myAttachmentService ;

    public DoSomethingWithAttachmentAction(MyAttachmentService myAttachmentService) {
        this.myAttachmentService = myAttachmentService;
    }

    public String doExecute() {
        myAttachmentService.someMethod();
    }
}

      

        
        
        
      

    

But then at runtime I get the following error:

Caused by: org.springframework.beans.factory.NoSuchBeanDefinitionException: No unique bean of type [com.atlassian.jira.bc.issue.attachment.AttachmentService] is defined: expected single matching bean but found 2: [attachmentService, myAttachmentService]
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:621)
    ...

      

        
        
        
      

    

How do I fix it and use my new component?