Find out why Squid isn't caching files

I’m running Squid in a Docker container, thanks to the b4tman/squid image on Docker Hub.

Environment

• Windows 10
• Docker Desktop Linux VM
• Testing with Chrome 85
• Configured proxy in Windows 10 whole-system proxy configuration

Squid Configuration

• I set the maximum_object_size 300 MB so that I could cache larger files.
• I also configured a filesystem cache using cache_dir ufs /var/cache/squid 2000 16 256
• Enabled large memory cache cache_mem 800 MB

However, when I try to download certain files, they aren’t being cached. I can tell, because the files are downloading again, on my slower internet connection.

I tried adding this line:

refresh_pattern -i .(iso|avi|wav|mp3|mp4|mpeg|swf|flv|x-flv)$ 43200 90% 432000 override-expire ignore-no-cache ignore-no-store ignore-private

Files That Are Cached

The files below ARE being cached by Squid, and download instantaneously.

• Any of the Node.js packages here

Files That Are NOT Cached

The following files are NOT being cached in Squid.

• Small (18-20 MB) MP3 audio and MP4 video files downloaded from MSDN Channel 9
• Medium (50 MB) .zip and .tar.gz files from GitHub releases – example

Question: How can I discover why Squid isn’t caching certain files, while other files are being properly cached?