In File/Board Setup, if the user chooses “Two layers, parts on front” or “Four layers, parts on front”, the layer set enabled does not include B.Mask. In my design there are no tracks on either F.Cu or B.Cu (only zones, tracks are on inner layers) so the solder mask is the same for the front and the back, so I told JLCPCB to just use the F.Mask file for both sides.
Shouldn’t there be a B.Mask layer if there are any THT parts on the board, or anything else which has pads on the bottom side?
Using KiCAD 5.1.4 Fedora 30 package.