Angular 8 can't download docx file from base 64 string

Question

I'm having problems downloading a .docx file from server. I can correctly upload and download pdf files or txt files, but not docx files.
When uploading I do this in my onNativeInputFileSelect method:

 if(event.target.value){
      const file: File = event.target.files[0];

      let fileToUpload = new MyFile();
      let filenames = file.name.split('.');

      fileToUpload.name= filenames[0];
      fileToUpload.type = filenames[1];


      this.changeFile(file).then((base64: string):any => {
        fileToUpload.content= base64;
       this.networkService.uploadFileDocumento(fileToUpload).subscribe(() => {
...

where changeFile is this method:

changeFile(file){
    return new Promise((resolve, reject) => {
        const reader = new FileReader();
        reader.readAsDataURL(file);
        reader.onload = () => resolve(reader.result);
        reader.onerror = error => reject(error);
    });
}

When I upload a pdf file, the base64string starts with

data:application/pdf;base64,JVBERi0xLjUKJb/3ov...

and when I upload the docx file the base64string starts with

 data:application/octet-stream;base64,UEsDBB...

Then I try to download the files. I can correctly download the pdf file, but when I try to download the docx file I don't get the correct file type (the type of file is "File") and I can't open it as a document. When I get the file from the server I do this:

let splitted = file.content.split(',');
  let type = splitted[0].substring(splitted[0].lastIndexOf(':') +1, splitted[0].lastIndexOf(';'));
  let blob = this.b64toBlob(splitted[1], type);
  let blobUrl = URL.createObjectURL(blob);
  let doc = document.createElement("a");
  doc.href = blobUrl;
  doc.download = file.name;
  doc.click();

Where b64toBlob is the following method:

b64toBlob(b64Data, contentType = '', sliceSize = 512) {
  const byteCharacters = atob(b64Data);
  const byteArrays = [];

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

    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);
  }

  const blob = new Blob(byteArrays, { type: contentType });
  return blob;
}

Why do I get application/octet-stream when uploading the file? And then I can't download id with the correct type? Where am I going wrong?