Use the Browse... Boutton 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 Kbytes.

The file content type must be one of those below:
application/force-download
application/msword
application/octet-stream
application/pdf
application/postscript
application/powerpoint
application/rar
application/vnd.ms-powerpoint
application/vnd.ms-powerpoint.presentation.macroEnabled.12
application/vnd.ms-pps
application/vnd.oasis.opendocument.text
application/vnd.openxmlformats-officedocument.presentationml.presentation
application/vnd.openxmlformats-officedocument.presentationml.slideshow
application/vnd.openxmlformats-officedocument.wordprocessingml.document
application/x-gzip
application/x-rar
application/x-rar-compressed
application/x-tex
application/x-zip
application/x-zip-compressed
application/zip
audio/wav
text/html
text/xml
text/plain
text/richtext
video/mp4

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