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 150000 KiB.

The file content type must be one of those below:
application/force-download
application/iopexport_bib
application/msword
application/octet-stream
application/postscript
application/pdf
application/vnd.ms-powerpoint
application/vnd.oasis.opendocument.text
application/vnd.openxmlformats-officedocument.wordprocessingml.document
application/x-tex
application/x-zip
application/x-zip-compressed
application/zip
image/jpeg
image/png
text/html
text/plain
text/richtext
text/x-tex

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