Here are some known scenarios where files will not upload to the Portal:

The file being uploaded currently resides within a .zip folder on the local machine.
FIX: unzip the local archive, then upload the uncompressed file(s).

The file being uploaded was not originally created as the current file type.
Such as you tried manually renaming a .txt file to a .pdf file.
FIX: change the file extension to the original type, then upload. If you need a different format file, first convert the file properly via applicable software, then upload.