Bug 471120

SUMMARY
When Kontact (installed via Flathub's Flatpak) has an email with an attachment (pdf, png), this attachment cannot be opened by double clicking on it in the message viewer. If the attachment is double clicked, the user is greeting with an "Open With" screen that does not show any system apps or other flatpaks. The apps it does show what appears to be internal "programs" to the flatpak itself (like kmail, kaddressbook). 

STEPS TO REPRODUCE
1. Install Kontact Flatpak via Flathub, Configure an email account that has emails with standard attachments (pdf, png, jpg)
2. Click on an email with an attachment in the message list.
3. Inside the message preview, double click on the attachment in either the header or within the email's body

OBSERVED RESULT
"Open with" window appears, but the appropriate installed apps are not in the list.

EXPECTED RESULT
The flatpak should respect the system's "file associations" and launch the correct corresponding mime-type app. 

SOFTWARE/OS VERSIONS
Operating System: Fedora Kinoite Linux 38
KDE Plasma Version: 5.27.5
KDE Frameworks Version: 5.107.0
Qt Version: 5.15.9
Kernel Version: 6.3.7-200.fc38.x86_64 (64-bit)
Graphics Platform: Wayland
Graphics Processor: AMD Radeon RX 480 Graphics
Kontact Ver: Version 5.23.2 (23.04.2)
Install Method: Flathub flatpak

ADDITIONAL INFORMATION
Tested against Evolution flatpak in the same environment. It respected the mime-types, so its unique to the Kontact flatpak vs a system-level issue.