The lookup of mimetypes is broken when using Go on Solus. I ran into this when running the tests inside https://github.com/fyne-io/fyne/tree/master/widget, where all the mime related tests are failing when running them here on Solus. Installing httpd and symlinking the mime types using sudo ln -s /usr/share/defaults/httpd/mime.types /etc/mime.types fixes it. This is because of how Go looks up the MimeType information. See https://github.com/golang/go/blob/9ea6364a5e9f776af36604c2c20501e6d07f8467/src/mime/type_unix.go#L19 for the available lines.
There are who possible issues at play here depending on how you look at it. I would say that the issue partly lies in Go and the other one in Solus. We are currently lacking a global mime type file for listing all the available mime types (such as /etc/mime.types, related to T8386). Go also does not support for example /usr/share/defaults/httpd/mime.types that our httpd package provides.
My suggestion is that we either add a global /etc/mime.types or a more stateless version in /usr/share/defaults/mime.types (the latter requiring a PR upstream to Go or patch downstream). This would avoid people needing to download httpd to get mime stuff working, which for obvious reasons is not a very user friendly solution.