@eremef
Thanks. It's not that simple. It's a registry problem. I had to run a run a .reg file like this. I have to do it again now that Cura v6 is out...
Windows Registry Editor Version 5.00
[HKEY_CLASSES_ROOT\.stl\OpenWithProgids]
".stl.4.13"=-
[-HKEY_CLASSES_ROOT\.stl.4.13]
[HKEY_CLASSES_ROOT\.stl]
@="Cura.model"
[HKEY_CLASSES_ROOT\Cura.model]
@="Standard Tessellation Language (STL) files"
[HKEY_CLASSES_ROOT\Cura.model\DefaultIcon]
@="C:\\Program Files\\Ultimaker Cura 5.2.1\\Ultimaker-Cura.exe,0"
[HKEY_CLASSES_ROOT\Cura.model\shell]
@="open"
[HKEY_CLASSES_ROOT\Cura.model\shell\open]
@="Open with Ultimaker Cura 5.2.1"
[HKEY_CLASSES_ROOT\Cura.model\shell\open\command]
@="C:\\Program Files\\Ultimaker Cura 5.2.1\\Ultimaker-Cura.exe \"%1\""
[-HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts\.stl]
[HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts\.stl\OpenWithProgids]
"Cura.model"=hex(0):