Tools - Options - LibreOffice - General - LibreOffice File Associations - Windows Default Apps
Pressing the "Windows Default Apps" button has no obvious effect.
Is that a bug?
(have set OS to Windows - but maybe the problem is more general?)
Version: 184.108.40.206.alpha0+ (x64)
Build ID: 61e1e0413296928d929f99c0f006c6cbbcf4ac40
CPU threads: 8; OS: Windows 10.0 Build 18363; UI render: GL; VCL: win;
Locale: en-US (en_DK); UI-Language: en-US
does not give any advice (and could not find anything from searching help).
The WIP release notes for 7.0 on wiki  has this item:
Windows Default apps button was added to Options ▸ LibreOffice ▸ General, to call Windows file associations management. This button behaves according to Microsoft file association management policy, which is to open "Default apps" on Windows 7, 8, and 8.1; and to show a message telling user how to open that applet manually in Windows 10. This only works with registered application, so would not work for portable or testing (e.g. beta, when not using WRITE_REGSITRY=1 installation option) releases tdf#44462 (Mike Kaganski, Collabora)
So it's probably expected that the button in daily build version doesn't do anything.
Thanks Ming. I will change Component to "Documentation"
Seth Chaiklin committed a patch related to this issue.
It has been pushed to "master":
tdf#130620 add help about File associations