oauth2中不受支持的授权类型问题

Question

我有一个web api应用程序，我需要配置 Oauth 来生成令牌：

 public void ConfigureAuth(IAppBuilder app)
        {

            app.CreatePerOwinContext(ApplicationDbContext.Create);
            app.CreatePerOwinContext<ApplicationUserManager>(ApplicationUserManager.Create);
            app.CreatePerOwinContext<ApplicationSignInManager>(ApplicationSignInManager.Create);
            app.CreatePerOwinContext<ApplicationRoleManager>(ApplicationRoleManager.Create);

            app.UseOAuthAuthorizationServer(new OAuthAuthorizationServerOptions
            {
                TokenEndpointPath = new PathString("/oauth/token"),
                Provider = new AuthorizationServerProvider(),
                AccessTokenExpireTimeSpan = TimeSpan.FromMinutes(30),
                AllowInsecureHttp = true,

            });
            app.UseOAuthBearerAuthentication(new OAuthBearerAuthenticationOptions()); 


        }
    }

     public class AuthorizationServerProvider : OAuthAuthorizationServerProvider
     {
         public override async Task ValidateClientAuthentication(OAuthValidateClientAuthenticationContext context)
         {
             context.Validated();
         }
         public override async Task GrantResourceOwnerCredentials(OAuthGrantResourceOwnerCredentialsContext context)
         {
             UserManager<IdentityUser> userManager = context.OwinContext.GetUserManager<UserManager<IdentityUser>>();
             IdentityUser user;
             try
             {
                 user = await userManager.FindAsync(context.UserName, context.Password);
             }
             catch
             {
                 // Could not retrieve the user due to error.  
                 context.SetError("server_error");
                 context.Rejected();
                 return;
             }
             if (user != null)
             {
                 ClaimsIdentity identity = await userManager.CreateIdentityAsync(
                                                         user,
                                                         DefaultAuthenticationTypes.ExternalBearer);
                 context.Validated(identity);
             }
             else
             {
                 context.SetError("invalid_grant", "Invalid User Id or password'");
                 context.Rejected();
             }
         } 

我使用Advanced Rest Client chrome扩展来测试令牌的生成，如下所示：

如何解决此问题？