Fail to open exported .xlsx file

On CiviCRM 5.82.0 and Drupal 7 when trying to export contacts from the standard search (now based on SearchKit)

a .xlsx file is generated and downloaded

Rechercher_des_contacts.xlsx

but it can't be opened properly (as it can't be opened there's no privacy issue)

(LibreOffice on Linux Debian)

(MS Office 365 on Windows)

It seems at first that it's an encoding issue (default is UTF-8) but actually no encoding option is giving a working version.

I've opened the file as an archive in the terminal

but I couldn't find anything useful in the .xml files

A simple workaround is to use the .csv file format which opens as expected but is less common for our client.

I just tried this on master & did not hit the same - although I DID hit the same opening your file.

I didn't find anything obvious on the phpoffice gitlab that relates https://github.com/PHPOffice/PhpSpreadsheet/issues?q=is%3Aissue%20state%3Aclosed%20extra%20byte

@bgm any input?

added sig:unverified-bug triaged labels

The one thing I notice is that there is a blank line in line 1 of the fie when I remove that line using a text editor of the .xlsx file, then it opens normally in excel.

I have found that this sort of thing can affect images being loaded on the site e.g. Contact Images as well.

This can be usually hard to find but generally the problem will be where its like in an extension's primary .php file or the civicrm.settings.php or similar file has <?php instead of <?php at the start of the file or a new line like the following.

<?php

This is causing the extra buffer to be outputted and therefor is confusing excel/libreOffice when loading it.

This seems very likely ^^

I used to monitor sites against this, because on WP sites it is somewhat frequent, and it's difficult to find the culprit.