Misidentification of small ATR boot images

[robmcmullen]

Creating a boot image and then examining it produces:

$ p run.py cent.atr boot -b centipede.xex 
XFD Disk Image (size=5585 (43x128B)
adding  $6000-$7581 ($1581 @ $0006) from centipede.xex
adding  $7ff8-$8000 ($0008 @ $158b) from centipede.xex
adding  $0600-$0634 ($0034 @ $1597) from centipede.xex
adding  $02e0-$02e2 ($0002 @ $15cf) from centipede.xex
saving boot disk cent.atr
$ p run.py cent.atr 
XFD Disk Image (size=6016 (47x128B)
cent.atr: BSAVE data $0300-$0a00 ($0700 @ $0004)

where it should be recognized as a short, valid ATR disk image at least because the header magic is correct.