I'm getting error
File Upload: No temporary directory was found for the file 'file_0_'
when trying to upload a small JPEG file.
I've set the archive folder and all its subfolders to 777.
I seem to have successfully negotiated php.ini related issues, and the "imagemagick directory path" test in the admin settings form reports success.
Any idea what I'm still missing?
PHP uploads files into a "temporary" directory first, before copying them to where they need to go permanently.
The error you're seeing means that PHP can't find or can't write to the temporary directory it's configured to use. This isn't an Omeka error, rather a PHP/server configuration error.
The php.ini directive
upload_tmp_dir lets you manually specify the path to the temporary directory. If it's left blank, the default is to try to use the server's temporary directory (generally /tmp).
What specific action you should take depends whether you are on shared hosting or your own server. Regardless, PHP needs to be pointed to a temporary directory that is web-server-writable.