[afc] Only check for a preview icon if actually requested
[afc] Only check for a preview icon if actually requested

Should speed up enumeration slightly.

halfline/gvfs,gicmo/gvfs,gicmo/gvfs,xkahn/gvfs-cmis,philipl/gvfs,philipl/gvfs,xkahn/gvfs-cmis,xkahn/gvfs-cmis,cpatulea/gvfs,halfline/gvfs,halfline/gvfs,xkahn/gvfs-cmis,gicmo/gvfs,cpatulea/gvfs,gicmo/gvfs,philipl/gvfs