More JAR file weirdness
OK, a kind visitor named Julius was tenacious enough to try several JAR ebooks — all of which failed with the same “INVALID FILE” error (great error, eh?).
Lots of messing about later I realized that the MANIFEST.MF file that must be created with each ebook has got an extra tab character at the beginning of each line; I had been a little overzealous when formatting my PHP scripts… Anyway, the JAR files seem to work OK again. I think. I’ll have to check with Julius :)