I think, you have to pass args as tuple:
s = mp.Process(target=get_md5, args=(file_1,))
You are missing the comma. If you miss the comma, individual characters in file_1 treated as separate args.
EDIT:
Including Adam Smith's response which i think appropriate to context:
It looks like Process
accepts *args
and **kwargs
, which means when you hand it r"C:\Users\Nick\Documents\production\TEST\\test.txt"
it iterates over it to give *args == ["C",":","\\","U","s","e","r","s","\\","N", ... ]