Coverting base64 image into file image is giving me string format files

I got this base64 image

data:image/jpeg;base64,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…", "data:image/png;base64,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

I am trying to save this base64 into the image and wanna save it in the server.

So I try to create a new file

const file = new File([base64Image], 'things', { type: 'image/png' });

let newObj: Record<string, File> = {
          signature: file,
      };

post(newObj)

But the thing is this is not save as .jpeg or .png

it is saving as this testing/382390e0-fdd1-4e70-91aa-31b36d6f8356.things

Shouldn't it be saving as testing/382390e0-fdd1-4e70-91aa-31b36d6f8356.jpeg

Here is the method I am using to append the data

export class FileService {
  async uploadFile(
    fileId: string,
    {
      documents,
      checkbox,
    }: { documents?: Record<string, File>; checkbox?: boolean }
  ): Promise<FileEntry> {
    const form = new FormData();

    Object.entries(documents || {}).forEach(([k, file]) =>
      form.append(k, file, file.name)
    );
    return ...
  }
}