uploading more files

I created a form with a file type field where the user should be able to upload multiple files, but it won’t let me do so. I put it like this:

 <label> Carica i tuoi documenti (fino a 25mb) (facoltativo)
    [file file-999 limit:25mb filetypes:pdf|jpg|jpeg|png|webp multiple]</label>

but it doesn’t work, it only lets me upload one.