[Linux/Flatpak] Attaching files via Drag-n-Drop or copy/paste clipboard does not work on some environments

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Problem

A file can be attached by drag-n-drop or copy/paste but it cannot be sent with an infinite load or an error.

Workaround from the user side

On your own risk you can provide read only access to the entire file system for the app via:

flatpak override --user --filesystem=host:ro com.nextcloud.talk

or only to the home and /tmp (for some screenshot tools) directories:

flatpak override --user --filesystem=home:ro --filesystem=/tmp:ro com.nextcloud.talk

It can be reverted with:

flatpak override --reset com.nextcloud.talk

Details

Flatpak uses portal org.freedesktop.portal.FileTransfer for file transfer from the host.
It is supported by Electron since v44 (Chromium 151). However, it requires support from both sides - the application and the host.

Currently it works on all GTK 4 file managers, for example, on GNOME's Nautilus, and on others like KDE's Dolphin.

Example distribution | Desktop Environment | Default FM | Status
-- | -- | -- | -- | --
Ubuntu 26 | GNOME 49 | Nautilus | ✅ Confirmed
Debian 13 | GNOME 48 | Nautilus | 🚧 Untested
Fedora 44 | KDE Plasma | Dolphin | 🚧 Untested
openSuse | KDE Plasma | Dolphin | 🚧 Untested
Linux Mint | Cinnamon | Nemo | ❌ Unsupported
Linux Mint | Xfce | Thunar | ❌ Unsupported

Proper workaround

Providing full filesystem access is against the sandboxing idea and cannot be shipped as a solution.

On the app side, file transfer support should be detected. If it is not available, the app should show a warning to the user on paste/drag with a suggestion to use the attach menu.

No source files or tests are named. Start by locating the attachment handling for drag-and-drop and clipboard paste, then inspect how Electron and the Flatpak FileTransfer portal are detected on supported and unsupported environments. Done means unsupported transfers produce a warning suggesting the attach menu, while supported transfers continue to work.