Overwritting thumbnail with url function strips original classes

Hi, first of all thanks for a great plugin!

I try to get the correct setup for both thumbnail resising using Jetpack and lazy loading using any 3rd party plugin. After some investigation I have noticed, that in order to work this properly, attachment-$size and size-$size classes have to be preserved in the original img tag generated by the theme.

But, your plugin function knawatfibu_overwrite_thumbnail_with_url strips all original img tag classes and replaces them with ‘external-img wp-post-image ‘.

Is it possible to modify the function in a way that the external-img class was added to existing classes? For example:

Original img tag classes: ‘attachment-my-thumbsize size-my-thumbsize wp-post-image’
Updated img tag classes: ‘attachment-my-thumbsize size-my-thumbsize wp-post-image external-img’

I am not a PHP expert and do not dare to modify the plugin code…

I hope I am clear, sorry for my English.