The database table for storing the filesize seems to be of a wrong data type.
You need to go into your PHPMyAdmin (i hope you have this as it makes this a lot easier) and go to the table downloads_files_records, click "Structure" and edit the column record_size.
Under attributes try setting it to unsigned, and then hit the "SAVE" button at the bottom.
Now try to upload your file.
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If that doesn't work, put the unsigned back to the blank option (the first one) and change the TYPE to VARCHAR and LENGTH to 10:
And see if that works.
I also hope you've set this option: