Both on discuss.pixls.us and on raw.pixls.us attempting to download a DNG file results in a file with a tif extension using Firefox (at least on Windows). Example response headers for GET https://raw.pixls.us/getfile.php/1022/nice/Adobe%20DNG%20Converter%20-%20Canon%20EOS%205D%20Mark%20III%20-%2016bit%20uncompressed%20(3:2).DNG:
HTTP/1.1 200 OK
Date: Fri, 13 Apr 2018 05:11:37 -0400
Server: Apache
X-Powered-By: PHP/7.4.21
Expires: Thu, 19 Nov 1981 08:52:00 GMT
Cache-Control: no-store, no-cache, must-revalidate
Pragma: no-cache
Content-Disposition: attachment; filename="Adobe DNG Converter - Canon EOS 5D Mark III - 16bit uncompressed (3:2).DNG"
Content-Length: 48382640
Keep-Alive: timeout=5, max=100
Connection: Keep-Alive
Content-Type: image/tiff
An example from discuss.pixls.us is Pentax K1-II, difference between Jpeg and DNG - #7 by Peter - https://raw.pixls.us/getfile.php/3345/nice/Pentax%20-%20K-1%20Mark%20II%20-%2014bit%20(3:2).DNG
Interestingly, some DNG files are not affected; for example, in the topic Revealing more refined details using RT, the file https://discuss.pixls.us/uploads/short-url/jW3sJuWd8LpRStYcxiSkGIAld1W.dng is downloaded with the dng extension. On raw.pixls.us, a DNG file with a lower-case .dng extension is also assigned the TIFF content type. GET https://raw.pixls.us/getfile.php/2835/nice/Apple%20-%20iPhone%208%20-%2016bit%20(4:3).dng also has response headers with that information.
Using Brave (a Chromium-based browser) on Windows saves those files with the original extension.
It was @Peter who brought this to my attention.