title: Argument of type 'void' is not assignable to parameter of type 'string' in Angular 6
body: <h2>Mycomponent</h2>
  
  <pre><code>  onFileChangedForCV(event) {
    this.details.= event.target.files[0]
    }
  candidateDetails(){
  
      let formData = new FormData();
      let photo= formData.append('profile_pic',this.details.photo);  
      let name=this.details.name;
      let age = this.details.age;
      let cv = this.details.cv;
      let experience = this.details.experience;
      let current_salary = this.details.current_salary;
      let expected_salary = this.details.expected_salary;
      let notice_period = this.details.notice_period;
      if (this.contentEditable){
       this.detaisservice.candidateDetailsExperienced(photo(Here throwing ERROR),name,age,cv,experience,current_salary,expected_salary,notice_period)
        .subscribe(
          data =&gt; {
              this.details.DetailsResponse = data;
                 if(this.details.DetailsResponse.code =='200'){ 
                   }
          },
          error =&gt; alert(error),
          () =&gt; console.log(this.details.DetailsResponse)
        );
      }
     }
  </code></pre>
  
  <h2>MyService File</h2>
  
  <pre><code> candidateDetailsFresher(photo:string,name:string,age:string,cv:string){
        let body = JSON.stringify({photo:photo,name:name,age:age,cv:cv});
        let headers = new Headers({ 'Content-Type': 'application/json'});
        let options = new RequestOptions({ headers: headers });
        console.log(body);
        console.log(options);
       return this.http.post('http://127.0.0.1:8000/api/seeker/candidate_details/', body, options)
       .map(this.extractData)
       .catch(this.handleError);
     }
    private extractData(res: Response) {
        let body = res.json();
        return body || [];   
      }
      private handleError (error: any) {
        return Observable.throw(error.json().error || 'Server error');
      }
  </code></pre>
  
  <p>Error:Argument of type 'void' is not assignable to parameter of type 'string'. It's showing error when i am trying  to call API in Angular 6 .Please help me to resolve this error.Ex:let photo = this.details.photo; (it working but i want to Send as FormData).</p>