What is an "author's novelty file" in journals?

It sounds like the journal wants you to identify in what manner your paper has expanded the state of knowledge in your field. Have you contributed a new experimental method? Or perhaps you've provided new insights into some sort of phenomenon, or measured data for a new material, or so on. These contributions should be collected in this "novelty file."

I recently submitted a paper to IJNME. In the 'File Upload' section, it states the following:

"Author's Novelty File" - All Authors are now required to supply an additional file. Please briefly describe, in a bullet pointed list, the new contributions of your paper to the field. Do not repeat for this purpose the content of your abstract. Instead, provide a brief itemized list of these contributions in a separate file.

I hope this helps.

I think they're asking for an "explanation for non-experts". Often, the abstract of a paper gives only enough information to be fully understood by an expert in the field (or at least in that problem). Part of your job in explaining your work is explaining why it's important. Think about how you would explain your work to a student with roughly the same background as you, but who hasn't thought much about your project. Now think about how you would explain it to a researcher working in a completely different field... in 5 sentences or less. Your novelty file should say something like that. Emphasize here's what we added.