.xlsx file gets corrupted/unreadable at client side when sent as response using Flask

Question

I have created a Flask app that receives XML file, processes it, saves it on my machine and sends that copy of .xlsx file as response.

The problem is whenever I am sending the .xlsx file, the requester gets the .xlsx file in unreadable format. Also, the file at my machine that is saved is 6kb while the requester receives 9kb file. I did the same with .CSV format and it works fine.

I am using Apache v2.4 and mod_wsgi v5.0 Python v3.12.1.

Following is my code:

import request
from flask import Flask, send_file, redirect

@app.route('/func0')

def func0():

     # Store XML in a directory
     return redirect_to_func1

@app.route('/func1', methods=['POST'])

def funct1():

     #.. Some processing
     resp =      send_file(os.path.join(path_to_file, filename), mimetype='application/vnd.openxmlformats-officedocument.spreadsheetml.sheet', as_attachment=True)

     return resp

I looked through everywhere tried all combinations. The thing is it worked with .CSV file while it not working with .xlsx file.