Only load plugins that are lua files

Before trying to load a plugin or checking its version verify if it
looks like a lua file.

Close issue #349.

data/core/init.lua

@@ -664,8 +664,8 @@ local function check_plugin_version(filename)
   if info ~= nil and info.type == "dir" then
     filename = filename .. "/init.lua"
     info = system.get_file_info(filename)
-    if not info then return true end
   end
+  if not info or not filename:match("%.lua$") then return false end
   local f = io.open(filename, "r")
   if not f then return false end
   local version_match = false
@@ -685,7 +685,7 @@ local function check_plugin_version(filename)
     end
   end
   f:close()
-  return version_match
+  return true, version_match
 end
 
 
@@ -700,13 +700,13 @@ function core.load_plugins()
     local files = system.list_dir(plugin_dir)
     for _, filename in ipairs(files or {}) do
       local basename = filename:match("(.-)%.lua$") or filename
-      local version_match = check_plugin_version(plugin_dir .. '/' .. filename)
+      local is_lua_file, version_match = check_plugin_version(plugin_dir .. '/' .. filename)
+      if is_lua_file then
         if not version_match then
           core.log_quiet("Version mismatch for plugin %q from %s", basename, plugin_dir)
           local ls = refused_list[root_dir == USERDIR and 'userdir' or 'datadir'].plugins
           ls[#ls + 1] = filename
-      end
-      if version_match and config.plugins[basename] ~= false then
+        elseif config.plugins[basename] ~= false then
           local modname = "plugins." .. basename
           local ok = core.try(require, modname)
           if ok then core.log_quiet("Loaded plugin %q from %s", basename, plugin_dir) end
@@ -716,6 +716,7 @@ function core.load_plugins()
         end
       end
     end
+  end
   return no_errors, refused_list
 end