I’m running Mac OS 10.11. Just tried to download the newest .dmg file at https://cryptomator.org/downloads/#macDownload. The link retrieved a 75MB file without an extension.
All of the other links appear to be behaving the same way.
That doesn’t sound right. 
Just tried the download here and it was as usual. Maybe it was a hiccup when you tried it? Does it still happen?
The download is not encrypted. If the dmg file extension is missing, add it manually. And verify the SHA256 checksum with
shasum -a 256 ~/Downloads/Cryptomator-1.3.1.dmg
in Terminal. It should be
d9649a87b084efd98ea65d6b93a95e0470b6778a3263fec786fddba818c7f140
for version 1.3.1.
Indeed, it doesn’t sound right! Yet that’s what happens 
I downloaded the file (which was identified as the actual hash tag), verified the checksum, added “.dmg” and … it opened as the expected .dmg. Strange.
I have the exact same problem on Windows 10 64 when using FireFox and trying to download the exe. However, the file downloads correctly using IE. I disabled all my FireFox add-ons but that doesn’t solve the issue but like I said the download works using IE.