How to send both text and binary data in axios post request?

I would need to find a solution to send via a single axios POST request both of the following:

1. json structure
2. binary file (excel file)

How can I achieve this?

  let files = event.target.files;
  const fileReader = new FileReader();
  fileReader.readAsText(files[0], null);
  fileReader.onload = () => {
    this.fileContent = fileReader.result;

  let binaryDataForObject = this.fileContent;

  let referenceDataStructure = {
    textData: textDataForObject,
    binaryData: binaryDataForObject,
    referenceDataFileExtension: this.referenceDataFileExtension,
    userProvidedDataTypes: this.columnTypes
  };
  }

  this.axios
    .post(
      "http://url,
      referenceDataStructure
  )

This works technically but on the java side I couldn’t figure out, how to decode the binary data (encoded as a string) so that it is treated as an excel file.

Thank You in advance for any meaningful responses.
Lubos.