How to fix downloading a corrupt excel file?

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C#

Javascript

I am trying to download an Excel file through the browser but when I open this file, I get the error in Excel that the file is corrupt.
If I save this file through the backend locally to a directory on my computer, I can open the file and see data in it with no issues. The corruption issue appears only when I download the file through the browser.

Below is what I have tried so far but still didn't manage to get it fixed.

Can someone help regarding this issue?
Thank you.

EDIT: This is a screenshot of the result of the "console.log(data)", maybe it will provide an extra clue about what is going wrong.

What I have tried:

For testing purposes and trying to fix the issue, I am just generating an empty Excel file based on a template in the backend, then passing it to the frontend.

File1.cs

using var workBook = XLWorkbook.OpenFromTemplate(templatePath);
`
`
workBook.SaveAs(memoryStream);

return memoryStream;
//In this class, I tried saving the file to a local directory, 
//and it works with no problem by using workBook.SaveAs(path here, etc.).
}

Controller.cs

 [HttpGet]
public async Task<IActionResult> SaveAction([FromQuery] int? propertyId)
{
`
`
stream.Position = 0;
stream.Flush();// tried without these 2 lines
return File(stream.ToArray(), "text/xls"); // I have tried this contentType // and this: "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet".
}

frontendFile.js
This is the js file. I also tried both contentTypes here, but neither changed the end result and file corruption issue.

$.get(_options.saveCoversUrl, function (data) {
                    console.log("file to frontend sent2");
                    console.log(data);
                    var a = document.createElement('a');
                    data = new Blob([data], { type: "text/xls" });
                    var url = window.URL.createObjectURL(data);
                    a.href = url;
                    a.download = 'sample.xlsx';
                    a.click();
                    window.URL.revokeObjectURL(url);