diff --git a/website/source/docs/builders/qemu.html.markdown b/website/source/docs/builders/qemu.html.markdown
index 651c69122..ab3935329 100644
--- a/website/source/docs/builders/qemu.html.markdown
+++ b/website/source/docs/builders/qemu.html.markdown
@@ -247,8 +247,10 @@ builder and not otherwise conflicting with the qemuargs):
     host port.
 
 -   `vm_name` (string) - This is the name of the image (QCOW2 or IMG) file for
-    the new virtual machine, without the file extension. By default this is
-    "packer-BUILDNAME", where "BUILDNAME" is the name of the build.
+    the new virtual machine. By default this is "packer-BUILDNAME.FORMAT", where
+    `BUILDNAME` is the name of the build and `FORMAT` matches the `format`
+    option, above. If you customize this you must specify the file extension or
+    none will be used.
 
 -   `vnc_port_min` and `vnc_port_max` (integer) - The minimum and maximum port
     to use for the VNC port on the host machine which is forwarded to the VNC