Thanks to thread
XnView MP > MP - General Support > Associating .bmp in Windows 8, utility
FileTypesMan by Nir Sofer and search function of Regedit I may advise you to try following solution.
Start Regedit, navigate to key
[HKEY_CLASSES_ROOT\Applications\xnview.exe\shell\open\command] and change default value of type REG_SZ of this key from
to
Hope this will help.
If association with XnView for .arw files won't catch up to changes, default value of
[HKEY_CLASSES_ROOT\.arw] key points to another place where "%1" has to be added. In my case default value is "ARW_auto_file", so it is key
[HKEY_CLASSES_ROOT\ARW_auto_file\shell\open\command] where default REG_SZ value must be set to
But I created association by checking "Always open this type of files with selected application". If you will use standard Windows 10 file types associator it will probably create some key like
[HKEY_CLASSES_ROOT\XnView.arw\shell\open\command]. Or maybe not, but some of XnView types have such keys.
P.S. Just something I found on the matter (not directly related to question).
MSDN: Application Registration states that developer can register file types supported by application with similar hierarchy of keys: HKEY_CLASSES_ROOT\Applications\mspaint.exe\SupportedTypes\.bmp and so on. In my Windows 10 registry in HKEY_CLASSES_ROOT\Applications\xnview.exe there was no SupportedTypes key. Maybe it is because I just copied whole folder from another place not using an installer. I wonder if I create SupportedTypes and add some rare image extensions to this key, will that let me select them in Windows standard association panel for XnView?
Update: answering myself, no it won't, application capabilities are registered in different way.