i usually put iso images on a "chmod +t"-ed directory.
9660srv refuses to mount such an image because it's "not a plain file".
well, it is just a plain file with QTTMP bit on.
Reference: /n/sources/patch/applied/9660srv-qttmp
Date: Sat Apr 16 08:23:36 CES 2005

--- /sys/src/cmd/9660srv/xfile.c	Sat Apr 16 08:23:19 2005
+++ /sys/src/cmd/9660srv/xfile.c	Sat Apr 16 08:23:14 2005
@@ -37,7 +37,7 @@
 	}
 	if((dir = dirfstat(fd)) == nil)
 		error("I/O error");
-	if(dir->qid.type != QTFILE)
+	if((dir->qid.type & ~QTTMP) != QTFILE)
 		error("attach name not a plain file");
 	for(fxf=0,xf=xhead; xf; xf=xf->next){
 		if(xf->name == 0){