When I work, I keep all my development local. If you develop locally using an Apache server, then you will most likely need to change the Apache user and group so that uploads (particularly through WordPress) will upload properly.
To change the user and group, find and edit this file:
Group configurations (usually set to
_www by default) and change them to the
Group of your Mac.
User _www Group _www
User your_username Group staff
Once completed, you will need to restart Apache.
sudo apachectl restart
Now uploads should work properly for you local WordPress install.