It looks like someone is trying to upload a file to your servers.The firewall is preventing this from actually happening -- but they shouldn't even be able to 'attempt' an upload, let alone get all the way to the point where it's just firewall that's blocking them.
My first advice would be to take a full back-up (files + DB) of your wordpress installation (if you haven't done so already), and keep the backup files somewhere other than this server.
Once you've safely backed up the files, look through your installation, specifically for files that have been modified recently, look at the entire wordpress folder structure (however deep it goes), because any .php file here will be executable. Look also for odd jpg/png/txt/gif/html files that might have be uploaded by attackers in the hopes of changing the extension to .php post-upload.
Although wordpress core has improved in security over the last couple years, their plugin ecosystem is still full of vulnerable code. My general rule is that the fewer plugins you have the better. So while your wordpress installation might be top-notch, a plugin somewhere might be giving them remote file upload capability, that they might have otherwise exploited were it not for the firewall.
Step 3, run a scan. One of my favorite tools is wp-scan It's a vulnerability scanner built specifically for wordpress, that scans your existing installation (including plugins and themes) and checks for known vulnerabilities across them all.
Step 4, update everything (if not done so already). Including Wordpress Core (which should update automatically). Run the scan after the updates to be sure.
The reason why you run a scan before updating, is that you might have a chance to detect the vulnerability pre-update, and that might give you an idea of what happened. If you update before scanning, the scan might turn up nothing.
In essence, the backup, inspect files, scan, update-and-rescan process should help ease some concerns you have.