Add support for files with multiple extensions (like .fb2.zip).

frontend/document/documentregistry.lua

@@ -14,9 +14,10 @@ end
 
 function DocumentRegistry:getProvider(file)
     -- TODO: some implementation based on mime types?
-    local extension = string.lower(string.match(file, ".+%.([^.]+)") or "")
     for _, provider in ipairs(self.providers) do
-        if extension == provider.extension then
+        local suffix = string.sub(file, -string.len(provider.extension) - 1)
+        if suffix == "."..provider.extension then
+        -- if extension == provider.extension then
             return provider.provider
         end
     end