Can't import multiple PDFs on Flatpak

[PhrozenByte]

Describe the bug
When trying to import multiple PDFs with Portfolio Performance on Flatpak, the application fails to read any file except the first selected file.

This issue is likely related to #4492, as it also seems to be an issue with the Flatpak document portal (maybe even the same issue).

To Reproduce
Steps to reproduce the behavior:

1. Install and run Portfolio Performance from a Flatpak
2. Select File, Import, PDFs and select multiple files from a directory
3. See error message dialog just showing the file's path from the Flatpak documents portal (e.g. /run/user/1000/doc/69a00cb1/my_file.pdf) as error message; there's such error for any selected file except the first; importing the first file works as expected

Expected behavior
Portfolio Performance should be able to import multiple PDFs at once on Flatpak installations as well.

A workaround is to first add all files to a ZIP archive and import that ZIP archive instead.

Screenshots
n/a

Desktop (please complete the following information):

• Arch Linux, GNOME Shell 47.4 (on Wayland)
• Flatpak 1.16.0
• Portfolio Performance 0.74.0 (from Flathub with Java 21.0.6+2)

[PhrozenByte]

Also see flathub/info.portfolio_performance.PortfolioPerformance#27