When using custom config files with the --config-file option combined with the -d (destination) flag, Kamal hardcodes the .yml extension for destination-specific config files, regardless of the base config file's extension.

Current Behavior

If you use a config file with .yaml extension:

kamal deploy --config-file my-app.yaml -d staging

Kamal will:

• Load my-app.yaml ✅ (works correctly)
• Look for my-app.staging.yml ❌ (hardcoded .yml extension)

This inconsistency means users must use different extensions for their base and destination files when using .yaml.

Expected Behavior

The destination config file should preserve the same extension as the base config file:

• Base file: my-app.yaml → Destination file: my-app.staging.yaml
• Base file: my-app.yml → Destination file: my-app.staging.yml

Root Cause

In lib/kamal/configuration.rb:42, the destination_config_file method hardcodes the .yml extension:

def destination_config_file(base_config_file, destination)
  base_config_file.sub_ext(".#{destination}.yml") if destination  # ❌ hardcoded .yml
end