The furnace flatpak doesn't have access to the host filesystem.

Selecting files to open and locations to write to is handled via "portals" which handles displaying a file chooser dialog and returning a virtual path to read/write (something like /run/user/1000/doc/9ba24df/music) without granting permission to any other path.

Currently furnace checks the path returned from a dialog and appends the correct extension, unfortunately this will write the file to the actual path /run/user/1000/doc/9ba24df/music.fur which doesn't make it to the selected host path.

Here are a list of issues I found whilst trying to come up with a good solution 😩 :

• Automatically add extension for save file samhocevar/portable-file-dialogs#43
• https://gitlab.gnome.org/GNOME/zenity/-/issues/96
• Missing default file name and extension in Save As flatpak/xdg-desktop-portal-gtk#191

I just checked and it seems to not be a problem in KDE.