[stable4.6] fix(attachments): Convert FileList to array for FireFox

[backportbot-nextcloud]

Backport of #5656

[codecov]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (151f50f) 15.67% compared to head (3cb5c1c) 15.66%.
Report is 1 commits behind head on stable4.6.

Files	Patch %	Lines
.../components/Editor/Attachments/AttachmentsList.vue	0.00%	2 Missing ⚠️
src/services/attachmentService.js	0.00%	1 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##           stable4.6    #5657      +/-   ##
=============================================
- Coverage      15.67%   15.66%   -0.01%     
=============================================
  Files            202      202              
  Lines           8634     8636       +2     
  Branches        1740     1740              
=============================================
  Hits            1353     1353              
- Misses          7281     7283       +2     

Flag	Coverage Δ
javascript	15.66% <0.00%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.