SELinux prevents writing to drupal filesystem directories even if they are set to 775 permission. This is because they are not in the right context. To be able to make apache upload files into them, run the following commands:
su - cd <drupal-install-path>/sites/default chown -R apache:apache files chmod -R 775 files chcon -R -t tmp_t files
Note that apache is the httpd service user name and be sure to replace <drupal-install-path> with your drupal installation path.