If a client implemented a policy to block (.mp3) file. And if today, a sample .pdf file has been converted as .mp3 file. The Menlo will still treat the file as a .mp3 file and the policy action will be "block".
.mp3 will still be downloaded instead of blocking because Menlo doesn’t use extensions, we look at the file itself to determine the file type.
Also when treating files, the proxy is not just taking the file extension into account but also the magic number, hence why it's still detected as a .mp3.