Does nextcloud block certain filename extensions?

Setup:

Nextcloud version (eg, 18.0.2) : 19.0.4
Operating system and version (eg, Ubuntu 20.04) : CentOS 7
Apache or nginx version (eg, Apache 2.4.25) : Apache 2.4.46
PHP version (eg, 7.1) : 7.3.24

The problem:

We are a Mac shop, and certain Mac apps generate files with extensions .dat, .config, and .key. I’m trying to set up a new Mac today, and none of those files will sync. I get an error:

“No E-Tag received from server, check Proxy/Gateway”

We don’t have a proxy/gateway, and we’ve confirmed that E-Tags are working—if I curl a copy of the file in my web root, I can see the E-Tag. And it’s only these three file extensions that won’t sync; everything else works fine.

Does anyone know if NextCloud blocks certain filetypes from sync for security reasons? (If so, this would have to be a recent change, as we’ve been using it for years without issue.)

thanks!

Never mind, problem resolved: A security file was blocking those filetypes.