Around Fedora 27, Nautilus stopped being able to open files with double click. It took me a very long time to figure out what was going on, but it turns out it was this bug:

[Fedora bug]: https://bugzilla.redhat.com/show_bug.cgi?id=674321
[Ubuntu bug]: https://bugs.launchpad.net/ubuntu/+source/exo/+bug/956255

I had to remove the following entries from ~/.local/share/applications/mimeapps.list , in the "[Added Associations]" section:

x-scheme-handler/http=exo-web-browser.desktop
x-scheme-handler/https=exo-web-browser.desktop
x-scheme-handler/file=exo-file-manager.desktop
x-scheme-handler/trash=exo-file-manager.desktop
x-scheme-handler/mailto=exo-mail-reader.desktop;

According to the Ubuntu bug, the core problem was fixed in this commit:

https://git.xfce.org/xfce/exo/commit/?id=b74caf97642ae2bf00a7754a5fb193f695b6f3d4

However, one commenter on the Fedora bug pointed out that "exo-0.6.2-3.fc16 is still taking the mime types from the X-XFCE-MimeType key and adding them to the [Added Associations] section of ~/.local/share/applications/mimeapps.list". This was still happening on a later version of exo (I don't know which version exactly, but maybe 0.10.x, since Nautilus broke for me around Fedora 27).

There are a few things that need to be fixed:

(1) exo needs to stop inserting these entries in mimeapps.list , if it is still inserting them

(2) exo needs to remove these entries from mimeapps.list, if they are found there, so that upgrading exo is enough to fix a broken system.

(3) One commenter on the Fedora bug pointed out that "I only noticed because some part of KDE was complaining about some lists in mimeapps.list not ending with ';' " -- exo needs to add a trailing ';' if it does need to add (corrected) lines to mimeapps.list.