ENH Add support for uploading multiple files to the file upload field

[rasstislav]

Issues

• Upload multiple files in File Upload Field #857
• Multiple images uploadfield #1242

Pull request checklist

• The target branch is correct
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• This change is covered with tests (or tests aren't necessary for this change)
• Any relevant User Help/Developer documentation is updated; for impactful changes, information is added to the changelog for the intended release
• CI is green