FWIW, when adding multiple files, in the file dialogue window, the order with which you select the files - e.g. the mouse pointer while holding Ctrl down - matters e.g. if you select chapter5.html then chapter1.html then chapter3.html they'll be added in that order in the book browser (look in the Name box in the open file dialogue, the files are added in the same order as they appear there).
IIUC, file names don't matter, it's how the files are ordered in the epub spine in the opf that matters.
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