Use the button to fill the field.

If your document consists of more than one file, you must encapsulate it in a ZIP arquive. In this case, the  'File Name'  field must be filled with the ZIP arquive name.

The file size must be less than 500000 KiB.

The file content type must be one of those below:
application/force-download
application/iopexport_bib
application/msword
application/octet-stream
application/pdf
application/postscript
application/rar
application/vnd.ms-excel
application/vnd.ms-powerpoint
application/vnd.ms-powerpoint.presentation.macroEnabled.12
application/vnd.oasis.opendocument.text
application/vnd.oasis.opendocument.text-template
application/vnd.openxmlformats-officedocument.wordprocessingml.document
application/vnd.openxmlformats-officedocument.wordprocessingml.template
application/x-rar
application/x-rar-compressed
application/x-tcl
application/x-tex
application/x-zip
application/x-zip-compressed
application/zip
image/bmp
image/gif
image/jpeg
image/pjpeg
image/png
image/tiff
text/css
text/html
text/plain
text/richtext
text/xml
text/x-tex

Filling this field is optional since you might want to update just the metadata.