Hello Dvořák Marek,
Welcome to the Microsoft Community.
As for your information, it seems that you want to insert a link connected to a local file on your computer. This is impossible since the link to your file need to be a network hyperlink, not a local file path. If you want your file to be connected within this loop page, you need to upload your file to a cloud storage space first. OneDrive would be a good option. After uploading, you can get an online path for your file. Paste it in the input box and you'll complete this process.
I hope you find the above information helpful. If you need any further assistance, please let me know and I'll provide more advice.
Best Regards,
William Yu | Microsoft Community Support Specialist