Change Header Content-Type from application/json to multipart/form-data

I have a project with next js. in this project I am using fetch for handling requests.

I wrote a wrapper for my requests as below

 fetch(url, {
        method: method.toUpperCase(),
        headers: 'Content-Type': 'application/json',
      ...headers,
    };,
        cache: cache,
        body: body
          ? JSON.stringify(body):null
      });

it works good for all requests with header Content-Type:’application/JSON

But I have an issue now I have a form and in this form, I have a file input so in this case my headers must be multipart/form-data.

I am using formData API but since I used application/JSON as the default header My data will be posted with the header of application/JSON. so I changed explicitly my headers for this specific request to multipart/form-data and it created another issue because in this way the browser doesn’t set boundary for this header.
this is my request

    const onSubmitHandler = (values) => {
       
            const formData = new FormData();
            formData.delete('captcha');
            formData.append('jobTitle', careerData.title);
            for (let value in values) {
                formData.append(value, values[value]);
            }
            setCaptchaIsRequired(false);
            const globalService = new GlobalService();

            globalService
                .postCareer(
                    formData,
                    { "Content-Type":"multipart/form-data" }
                )
                .then((res) => {
                    //some codes
                }).catch(e => console.log(e, 'ERRROR'));
        }
    };

I want to know how can do this and if this hasn’t any solutions is it Ok to set boundary my self or not

Thank’s all of you in advanced