Skips files with unknown extensions when not forcing a format.

Fixes #3685

agent/config/builder.go

@@ -231,6 +231,17 @@ func (b *Builder) Build() (rt RuntimeConfig, err error) {
 		src.Format = FormatFrom(src.Name)
 		if configFormat != "" {
 			src.Format = configFormat
+		} else {
+			// If they haven't forced things to a specific format,
+			// then skip anything we don't understand, which is the
+			// behavior before we added the -config-format option.
+			switch src.Format {
+			case "json", "hcl":
+				// OK
+			default:
+				// SKIP
+				continue
+			}
 		}
 		if src.Format == "" {
 			return RuntimeConfig{}, fmt.Errorf(`config: Missing or invalid file extension for %q. Please use ".json" or ".hcl".`, src.Name)

agent/config/runtime_test.go

@@ -361,6 +361,21 @@ func TestConfigFlagsAndEdgecases(t *testing.T) {
 				rt.DataDir = dataDir
 			},
 		},
+		{
+			desc: "-config-format disabled, skip unknown files",
+			args: []string{
+				`-data-dir=` + dataDir,
+				`-config-dir`, filepath.Join(dataDir, "conf"),
+			},
+			patch: func(rt *RuntimeConfig) {
+				rt.Datacenter = "a"
+				rt.DataDir = dataDir
+			},
+			pre: func() {
+				writeFile(filepath.Join(dataDir, "conf", "valid.json"), []byte(`{"datacenter":"a"}`))
+				writeFile(filepath.Join(dataDir, "conf", "invalid.skip"), []byte(`NOPE`))
+			},
+		},
 		{
 			desc: "-config-format=json",
 			args: []string{