A file past one of the reader's limits answers ERR_TOO_LARGE rather than ERR_UNREADABLE, and helm_last_error says which limit. A file that is not a .mul is one somebody picked by mistake; a file past a limit is one helm will not read however long it is looked at, and a page has to be able to tell somebody which. helm.mjs puts them on the thrown error as `.tooLarge` and `.detail`, and the boundary goes to version 4.