Totally weird behavior trying to download a Mac DMG file

So I've added a product to my online store that's in .DMG format. Most  
of the other files are ZIP or PDF. When someone completes a purchase,  
it downloads the file to them, and this works great:

header('Content-Type: application/octet-stream');
header('Content-Disposition: attachment; filename="'. 
$file_row["filename"].'"');
$size = filesize('../../store/files/'.$file_row['filename']);
header('Content-Length: '.$size);
readfile('../../store/files/'.$file_row['filename']);

So I have my DMG file. I've verified that it automounts and behaves  
friendly. If you do a direct download, it mounts on the desktop  
perfectly, and there's all the stuff inside.

However, when I complete a test purchase and download using the above  
code, the DMG file downloads, but then it mounts; the contents are  
copied into the Downloads folder; the image unmounts; and then  
deletes. All the contents are delivered, but not in a desirable way.  
You can see the status change in the Downloads window:

Mounting the image
Copying the image
Unmounting the image
Cleaning up

WTF?????