[SalesForce] Adding jQuery plugin as static resource

Turns out when I repackaged the zip I created it with a root directory, for a structure like this:

Root/image.png
...
Root/image-n.png

whereas the original had a flat list:

image.png
...
image-n.png

So of course the files were no longer found. Obvious in retrospect -- but I'll leave the question up in case it helps someone who makes the same mistake...

The "value" attribute is not valid for a <script> tag; you're thinking of the Visualforce <apex:includeScript> tag. Choose one or the other; if you use the HTML tag then the attribute you want is "src". That's why you get the error; your script is not getting loaded, and thus "mCustomScrollbar" is not written into jQuery.fn (the [object Object] cited in your error is actually a jQuery result set object, which can't find a "mCustomScrollbar" property in its inherited jQuery.fn prototype if that script file didn't load).

The use of jQuery.fn.noConflict, and avoiding usage of the global "$" reference are both good practices -- however these should not be necessary under most circumstances, even in Visualforce pages. One exception is when you use apex's tab component, which unfortunately imports an old Prototype script that also wants the global "$" symbol.