Dont think upload of multiple files can be achieved with File Input Field.
three workarounds i can think of
1) when uploading multiple files, advise users to upload a zip file, then on server sidethe zip file can be unzipped.
2) list the files in a table format. have checkbox next to filenames, users can then select multiple check boxes to select multiple file to upload
3) list the files in tree format and user can select the list of multiple files then system can upload the files
hopefully one of them works for you :)
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