Large number of "Additional Files" skipped during import

Just tried to import about 20k files into my library using the copy method. The files are almost all JPG, with a handful of .aae files and the occasional PNG. Mylio is reporting 19k skipped files. About 18k of those are under the “Additional Files” heading in the skipped files dialog. How can I determine what those were? FWIW, Clicking “Show Files” reveals a folder for unsupported files from that import, but it only contains about 50 .aae files.

Thanks in advance.