How do I sort a playlist by filename in iTunes?
Unfortunately this is not possible. As there isn't a column available for filename, one can't sort by it.

If you're willing to take the time, you can input the metadata and sort by track number. You'd only really need to enter the track numbers incrementally (like the file names), but you might instead choose to number the tracks per disk and then input which disk they're on as well. This way would need to be sorted by Album rather than track number, as it would otherwise go 1,1,1,2,2,2,3,3,3,… rather than 1,2,3,…1,2,3,…1,2,3,….

Sorry it's so inconvenient, but I hope this works for you. :)