Use empty string as default extension

For treating document settings it is better to use empty string ""
rather than "txt" as a default extension type.

settings.lua

@@ -57,8 +57,7 @@ function DocSettings:open(docfile)
 			end
 
 			if stored.highlight ~= nil then
-				local ext = string.match(docfile, ".+%.([^.]+)")
-				local file_type = ext and ext:lower() or "txt"
+				local file_type = string.lower(string.match(docfile, ".+%.([^.]+)") or "")
 				if file_type == "djvu" then
 					stored.highlight.to_fix = {"djvu invert y axle"}
 				end