How to construct the response body for InputStream which has an Excel sheet file?

I am writing a get API for getting excel sheet file stored in src/main/resources.

The InputStream has the content of excel file (xlsm format). But the response is having corrupted data.

@GetMapping("/downloadTemplate")
public ResponseEntity<?> downloadTemplate() throws IOException {
    String fn = "filename";

    try {
        File file = new File(getClass().getClassLoader().getResource(fn).getFile());
        InputStreamSource iss = new FileSystemResource(file);

        return ResponseEntity.ok()
                .header(HttpHeaders.CONTENT_TYPE, MediaType.APPLICATION_OCTET_STREAM_VALUE)
                .header(HttpHeaders.CONTENT_DISPOSITION, "attachment; filename= SimpleStandardTemplate.xlsm")
                .body(IOUtils.toByteArray(iss.getInputStream()));
    } catch (Exception e) {
        log.error("An error occurred while trying to downloadTemplate: {}", e);
        throw new ApiException(ApiErrorCode.DEFAULT_410, "Error while downloading");
    }
}

Seems like this line has a problem "body(IOUtils.toByteArray(iss.getInputStream()));" . If yet any alternate way? Else what could be the issue?

Edit: I think the issue is with content length.