imageRules property
Which encoding rules the slot is under, because Play's are not uniform:
the icon is the one image in either store that is asked for an alpha
channel ("32-bit PNG (with alpha)") while everything beside it is refused
one ("JPEG or 24-bit PNG (no alpha)"). Carried by the spec rather than
decided at the call site, so the exception lives next to the slot it
belongs to instead of in an if somebody has to remember.
Implementation
final StoreImageRules imageRules;