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

The file you are attempting to upload currently resides within a .zip folder on your local machine.
FIX: unzip the local archive, then upload the uncompressed file(s).

The file you are attempting to uploade 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.