1) Directories are all set at 0777.
2) Directories are owned by my server user (as opposed to the web server user, aka 'nobody').
3) PHP settings...could I be missing something there? I do have PHP set to allow uploads. I did notice that the PHP parameter upload_tmp_dir is unset, so I fixed that but I still get the problem. Any other PHP settings I should look for?
4) Could it be something in .htaccess preventing the file upload? (Although I don't think that's it.)
I can post more info as needed. OS is CentOS 7, web server is Apache 2.4.6, PHP is latest stable version (5.6.8).
ErrorException: copy(/var/www/......./internal_data/attachments/0/333-b62fd8aa3cd577ff23a4290a595391a0.data): failed to open stream: Permission denied - library/XenForo/Helper/File.php:253