Add DNG (`image/x-adobe-dng`) mimetype

⚠️ This issue respects the following points: ⚠️ • This is a bug, not a question or a configuration/webserver/proxy issue. • This issue is not already reported on [Github]( OR [Nextcloud Community Forum]( _(I've searched it)_. • Nextcloud Server is up to date. See [Maintenance and Release Schedule]( for supported versions. • I agree to follow Nextcloud's [Code of Conduct]( Bug description When clicking to download ".DNG" files, the get request gets content-type: image/x-dcraw which is according to [this]( is a general mimetype for raw files. On linux firefox only for some reason this results in the file getting the wrong extension of ".bay" instead of the ".DNG". I dont know why firefox prefers the contenttype over the content-disposition filename, and why only on linux (or maybe ff <=132.0.1?) and i guess that maybe is a firefox bug, but anyway the more specific mimetype for .DNG is image/x-adobe-dng If you want to…