fix image preload not accepting all images types
fix image preload not accepting all images types

if a file is not an image, it could still be a sound file (or something else
with might allow to preload in the future) so i can't loudly complain.
jrburga/jsvgdl,jrburga/jsvgdl,GameJs/gamejs,GameJs/gamejs,jrburga/jsvgdl,jrburga/jsvgdl,GameJs/gamejs