New Movie Video Import fails to load SWF files

Issue Summary

Despite it's origin, SWF is now an "open" format (the specification is open at least). I wanted to use this since I'm currently working with Animate CC 2020.5 on my current job, but it was not possible to import anything from the SWF's we produced.

Is it a limitation of FFMPEG? or something missing for its configuration?

Expected Results

The movie formats displayed as available for import should work as implied.

Video or Image Reference

ffmpeg path:
E:/Downloads/Software/Animation Apps/Pencil2D Builds/pencil2d-win64-2020-11-24/Pencil2D/plugins/ffmpeg.exe
ffprobe path:
E:/Downloads/Software/Animation Apps/Pencil2D Builds/pencil2d-win64-2020-11-24/Pencil2D/plugins/ffprobe.exe
System Info
Pencil2D version: 0.6.5 (nightly)
Build ABI: x86_64-little_endian-llp64
Kernel: winnt, 10.0.18363
Operating System: Windows 10 (10.0)
end

Samples

Included Samples (Bouncing Ball) These samples include only basic elements that should be read universally by any SWF loader / importer such as graphic symbols and motion tweens.

Flash MX (Flash Player 6) [This one is different because the format type changed after Flash CS5)
Flash CS6 (Flash Player 11.4)
Animate CC 2020.5 (Flash Player 32)

Pencil2D_SWF_Samples.zip

Steps to reproduce

Create a new Pencil2D file
go to File > Import > Movie Video
Browse for a standard SWF file (authored in Flash or Animate CC. I'll provide some below)
You'll get the error.

System Information

Pencil2D Version: Nightly Build Nov 24 2020

commit: 889f226aaf65ac2b4da00cb3f355b6b29fd5d571 date: 2020-11-24_17:31:44

Operating System: Windows 10 (10.0) CPU Architecture: x86_64 Qt Version: 5.12.10

pencil2d / pencil