The "multiple" attribute is a new HTML5 feature.
It allows the user to select multiple files to upload.
https://www.w3.org/TR/html-markup/input.file.html

The "multiple" attribute is at the moment not supported.
That will require a change

• in the MultipartRequestFilter or/and CosMultipartRequestFilter, both are not accepting the "multiple" option and not correctly pasring the request.
• in the form validation, e.g. maxFileSize (FileUplaodValidator)

It would be nice to upgrade the fileUpload field or provide a new multipleFileUpload field