Use multiple file upload for boefje handler

[noamblitz]

Changes

Upload multiple files at once instead of looping over them in boefje handler.

Issue link

Also fixes this bug #3570 funnily enough

Closes: #3570

Demo

Please add some proof in the form of screenshots or screen recordings to show (off) new functionality, if there are interesting new features for end-users.

QA notes

Please add some information for QA on how to test the newly created code.

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Code Checklist

• All the commits in this PR are properly PGP-signed and verified.
• This PR only contains functionality relevant to the issue.
• I have written unit tests for the changes or fixes I made.
• I have checked the documentation and made changes where necessary.
• I have performed a self-review of my code and refactored it to the best of my abilities.

• Tickets have been created for newly discovered issues.
• For any non-trivial functionality, I have added integration and/or end-to-end tests.
• I have informed others of any required .env changes files if required and changed the .env-dist accordingly.
• I have included comments in the code to elaborate on what is not self-evident from the code itself, including references to issues and discussions online, or implicit behavior of an interface.

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Checklist for code reviewers:

Copy-paste the checklist from the docs/source/templates folder into your comment.

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Checklist for QA:

Copy-paste the checklist from the docs/source/templates folder into your comment.

[ammar92]

Created but unused (yet)?

[noamblitz]

I'm going to close this PR since the bug was fixed by donny and this can be done later.