Magento2 handling of downloadable product filetypes

Question

If I offer downloadable products on Magento2, some of them are corrupted after download, I cannot open them. It sais things like "this zip

Working fine: PDF, EPUB

Corrupted file after download: ZIP, MOBI.

I have not tried other filetypes. Any idea how to fix that?

Magento 2.3.5-p1, PHP7.3 on Ubuntu 20.04

Answer 1

Solution found: I had empty lines in the beginning of my registration.php of my custom theme before the start of the opening <?php tag.

Outch.