dload.c does
handle->dlcb(filename, 0, ust.size);
even when we are resuming a file at a given offset, because that's the only way to tell cb_dl_progress to re-initialize its counters when we are starting a file download.

There are probably many ways to fix this, just unsure which one to choose :
1) track last filename in cb_dl_progress to detect new files and reset counters
2) add a parameter or abuse existing parameters (e.g. filename == NULL) to tell cb_dl_progress to reset
3) add a new callback function to reset or init a new file download at a given offset