Django dynamic file output on form submit doesn't work on mobile

I'm trying to output a file by writing to io.BytesIO and send it when form is submitted as file attachment. Everything works fine in Firefox on Linux, but mobile browsers send a GET request when download is accepted in browser and save the HTTP response as the downloaded file.

Here is my view function:

from django.views.decorators.csrf import csrf_exempt
from django.http import FileResponse, HttpResponse
import io


@csrf_exempt
def download_file(request):
    if request.method == 'POST':
        buffer = io.BytesIO()
        buffer.write('Text file content'.encode('UTF-8'))
        buffer.seek(0)
        return FileResponse(buffer, as_attachment=True, filename='file.txt')
    return HttpResponse('<form method="post"><button>Download</button></form>')

This is what the logs look like when form is submitted from Firefox on Linux:

[20/Sep/2020 18:15:31] "POST /test/ HTTP/1.1" 200 17

Downloaded file on Linux:

Text file content

This is what the logs look like when form is submitted from Firefox on Android:

[20/Sep/2020 18:16:47] "POST /test/ HTTP/1.1" 200 17
[20/Sep/2020 18:16:48] "GET /test/ HTTP/1.1" 200 52

Downloaded file on Android:

<form method="post"><button>Download</button></form>

I am using Python 3.8.5 and Django 3.1.1.

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