skip format check for filenames ending with dot

also add a unit test for func FormatStringFromFilename to cover such case

test/format_test.go

@@ -0,0 +1,36 @@
+package test
+
+import (
+	"testing"
+
+	"github.com/mikefarah/yq/v4/pkg/yqlib"
+)
+
+// only test format detection based on filename extension
+func TestFormatStringFromFilename(t *testing.T) {
+	cases := []struct {
+		filename string
+		expected string
+	}{
+		// filenames that have extensions
+		{"file.yaml", "yaml"},
+		{"FILE.JSON", "json"},
+		{"file.properties", "properties"},
+		{"file.xml", "xml"},
+		{"file.unknown", "unknown"},
+
+		// filenames without extensions
+		{"file", "yaml"},
+		{"a.dir/file", "yaml"},
+		{"file.", "yaml"},
+		{".", "yaml"},
+		{"", "yaml"},
+	}
+
+	for _, c := range cases {
+		result := yqlib.FormatStringFromFilename(c.filename)
+		if result != c.expected {
+			t.Errorf("FormatStringFromFilename(%q) = %q, wanted: %q", c.filename, result, c.expected)
+		}
+	}
+}