make a Post correctly with HttpModule of angular 5

I always receive the error 500

import { Injectable } from '@angular/core';
import { HttpRequest, HttpHandler, HttpEvent, HttpInterceptor } from 
'@angular/common/http';
import { Observable } from 'rxjs/Observable';
import { AuthService } from './auth.service';

@Injectable()
export class TokenInterceptor implements HttpInterceptor {

constructor(public auth: AuthService) {}

intercept(request: HttpRequest<any>, next: HttpHandler): 
Observable<HttpEvent<any>> {

request = request.clone({
  setHeaders: {
    Authorization: 'Bearer ${this.auth.getToken()}',
    'Content-Type':'multipart/form-data'
  }
});

return next.handle(request);
}
}

import { Component, OnInit } from '@angular/core';
import { HttpClient, HttpHeaders } from '@angular/common/http';
import { SelectItem } from 'primeng/api';
import { URL_SERVICIOS } from '../../config/config';

@Component({
selector: 'app-actualizarinfo',
templateUrl: './actualizarinfo.component.html',
styleUrls: ['./actualizar.component.css']
})
export class ActualizarinfoComponent implements OnInit {
genero: SelectItem[];
avatar: string;
headers: HttpHeaders
formdata = new FormData();

constructor( private http: HttpClient ) {
this.genero = [];
    this.genero.push({label: 'Seleccionar género', value: ''});
    this.genero.push({label: 'Masculino', value: 'Masculino'});
    this.genero.push({label: 'Femenino', value: 'Femenino'});
    this.avatar = JSON.parse(sessionStorage.getItem('avatar'));
}

enviar() {
console.log('enviar');

***this.http.post( URL_SERVICIOS + '/user/updateProfilePhoto', 
this.formdata.get('avatar'), { headers: this.headers}  )
.subscribe( resp => console.log( resp ));***


 }

change($event) {
console.log( 'se activo' );
this.formdata.append( 'avatar', $event.srcElement );
}

ngOnInit() {
}

}