[BUG] Dash upload components behaves differently on multiple file upload via drag & drop and file selector

[Fenigor]

Ccontext
A basic minimalistic dash app with an upload component that support multiple files upload of type ".pdf" on windows 11, but developing with wsl ubuntu 22

dash                 4.0.0
dash-core-components 2.0.4
dash-html-components 2.0.0
dash-table           5.0.0

• if frontend related, tell us your Browser, Version and OS

  • OS: Windows
  • Browser: Chrome and Edge
  • Version 144 and 146

Describe the bug

When dragging multiple files and dropping them on the upload component, only the first one triggers the callback.

Expected behavior

Both files to be sent in the callback in the backend

[camdecoster]

Thanks for the bug report! Could you please provide an example app that demonstrates the problem?

[SilberWilbrt]

Dash multiple upload reproduction

I can reproduce this issue with the attached minimal Dash app.

Environment:

• Windows
• Microsoft Edge
• Dash 4.4.0
• Python application running directly on Windows

I dropped three files at the same time, but the callback received only one:

filename count: 1
contents count: 1

Using the native file selection dialog and selecting the same three files works correctly.

I also confirmed the same behavior in a Django application running under WSL:

• multi-file drag-and-drop sends only one file
• selecting multiple files through the file dialog sends all files

The minimal reproduction uses only dcc.Upload(multiple=True) and contains no Django or application-specific code.

dash_multiple_upload_repro.py
requirements_dash_multiple_upload_repro.txt

[AnnMarieW]

Thanks for the example @SilberWilbrt
This worked in dash v3.4, but not dash v4.0 It looks like a regression introduced in #3481 when folder support was added.

[robertclaus]

@AnnMarieW thank you for the PR! @KoolADE85 can you please take a look and review this week?