Problem sending base64 encoded audio file via FormData

I am working on an Ionic application that uses Capacitor to record audio and then transcribe it to the server using OpenIA API. The audio recording is successful, and I get a base64 encoded file with the following structure:

{
  recordDataBase64: "BASE64_STRING",
  msDuration: 10000,
  mimeType: "audio/webm" // can be wav, mp3 or webm
}

Now I am trying to send this audio file to my server via a POST request using FormData in Ionic with react. I have implemented the following code:

class Utils {
  static async transcribe(value: {
    recordDataBase64: string;
    msDuration: number;
    mimeType: string;
  }): Promise<string | null> {
    try {
      const fileName = 'recording.' + value.mimeType.split('/')[1];
  
      // Crear un objeto FormData
      const formData = new FormData();
  
      // Decodificar la cadena base64 a un Blob
      const blob = this.base64toBlob(value.recordDataBase64, value.mimeType);
  
      // Adjuntar el Blob al FormData con un nombre de archivo "file"
      formData.append('file', blob, fileName);
  
      // Realizar la solicitud POST
      const response = await ApiRequest('/transcribe', {
        method: 'POST',
        body: formData,
        headers: {
          'Content-Type': 'multipart/form-data; boundary=----WebKitFormBoundary7MA4YWxkTrZu0gW',
        },
      });
  
      // Obtener la transcripción del cuerpo de la respuesta
      const transcription = await response.json();
  
      console.log(transcription);
  
      return transcription;
    } catch (error) {
      console.error('Error al transcribir el audio:', error);
      return null;
    }
  }
  
// Función para convertir una cadena base64 a un Blob
static base64toBlob(base64: string, mimeType: string): Blob {
  const byteCharacters = atob(base64);
  const byteArrays = [];

  for (let offset = 0; offset < byteCharacters.length; offset += 512) {
    const slice = byteCharacters.slice(offset, offset + 512);

    const byteNumbers = new Array(slice.length);
    for (let i = 0; i < slice.length; i++) {
      byteNumbers[i] = slice.charCodeAt(i);
    }

    const byteArray = new Uint8Array(byteNumbers);
    byteArrays.push(byteArray);
  }

  return new Blob(byteArrays, { type: mimeType });
}
}

However, I am getting undefined as a response from the server, but when I use PostMan to test it works correctly.

the first log is using the webapp, the last is using postman

Can anyone help me identify why I am getting undefined and how I can correct this?