AFAIK there is only solution to this problem: you have to modify source code (I
am using version 3.46).

Although Httrack uses Content-type header for determination
of file type. You have to modify htslib.c file - hts_mime
array. In the case of pjpeg you could add:

{"image/pjpeg","jpg"},

If this table was configurable, it would be nice feature.