Strange thing. Once I downloaded one file from site given by xraale, then using "Show in folder" in browser, all .xls (including Lighthouse.xls) are displayed in Viewer Pane. But if try to display them from Libraries/Pictures (Libraries/Obrazy in my installation), situation is as described earlier.
Seems like it makes a difference whether they are in lister lib://Pictures (Viewer Pane does not show them) or in lister C:\Users\Username\Pictures (Viewer Pane shows them). Even if it is physically the same location.
I have seen program that was unable to write to Libraries\Documents, but that was able to write to Libraries\Documents\My Documents (that is physically the same location).