Use Set() for image file filtering and add .jpeg to default extensions

[playmobil42]

Changes:

• Instead of parsing config for each image file, create a Set() and use that to check image file extensions.
• Add .jpeg to default set of accepted image file types.
• ES6: Change some instances of var to const/let in node_helper.js.
• Remove some unneeded definitions of 'self'.
• Minor spelling correction and whitespace fixes.

Tested manually.