Last week a came across this error popup message when uploading a file throught the cck uploadfield. After searching on the web, i found out that a view people experience this problem in different situations with there Drupal website. Not every fix seems to work but this one did it for me, so i thought i'd share it with you.
To fix this problem i first updated my cck filefield module to the latest version.
Add the following to your htaccess file
php_value post_max_size 5M
php_value upload_max_filesize 5M
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