Try uploading the file to somewhere not in the website. So, if your website is installed in wwwroot, put it in the same folder as wwwroot, not in it. If it's removed from there, you can be sure that mojoPortal isn't removing it. Again, mojoPortal doesn't have any code to remove DLLs.
Can you check with your hosting provider to see if the server is falsely detecting the file as a virus?