diff --git a/internal/highlight/highlight.go b/internal/highlight/highlight.go
index e0f26f5572b7..640c0aee0209 100644
--- a/internal/highlight/highlight.go
+++ b/internal/highlight/highlight.go
@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"html/template"
 	"net/http"
+	"path"
 	"strings"
 	"time"
 	"unicode/utf8"
@@ -165,6 +166,8 @@ func Code(ctx context.Context, p Params) (h template.HTML, aborted bool, err err
 		stabilizeTimeout = 30 * time.Second
 	}
 
+	p.Filepath = normalizeFilepath(p.Filepath)
+
 	resp, err := client.Highlight(ctx, &gosyntect.Query{
 		Code:             code,
 		Filepath:         p.Filepath,
@@ -510,3 +513,23 @@ func splitHighlightedLines(input template.HTML) ([]template.HTML, error) {
 
 	return lines, nil
 }
+
+// normalizeFilepath ensures that the filepath p has a lowercase extension, i.e. it applies the
+// following transformations:
+//
+// 	a/b/c/FOO.TXT → a/b/c/FOO.txt
+// 	FOO.Sh → FOO.sh
+//
+// The following are left unmodified, as they already have lowercase extensions:
+//
+// 	a/b/c/FOO.txt
+// 	a/b/c/Makefile
+// 	Makefile.am
+// 	FOO.txt
+//
+// It expects the filepath uses forward slashes always.
+func normalizeFilepath(p string) string {
+	ext := path.Ext(p)
+	ext = strings.ToLower(ext)
+	return p[:len(p)-len(ext)] + ext
+}
diff --git a/internal/highlight/highlight_test.go b/internal/highlight/highlight_test.go
index 7d263480e8fa..746634f123d5 100644
--- a/internal/highlight/highlight_test.go
+++ b/internal/highlight/highlight_test.go
@@ -218,3 +218,50 @@ line3`
 		t.Fatalf("wrong highlighted lines: %s", diff)
 	}
 }
+
+func Test_normalizeFilepath(t *testing.T) {
+	tests := []struct {
+		name  string
+		input string
+		want  string
+	}{
+		{
+			name:  "normalize_path",
+			input: "a/b/c/FOO.TXT",
+			want:  "a/b/c/FOO.txt",
+		},
+		{
+			name:  "normalize_partial_path",
+			input: "FOO.Sh",
+			want:  "FOO.sh",
+		},
+		{
+			name:  "unmodified_path",
+			input: "a/b/c/FOO.txt",
+			want:  "a/b/c/FOO.txt",
+		},
+		{
+			name:  "unmodified_path_no_extension",
+			input: "a/b/c/Makefile",
+			want:  "a/b/c/Makefile",
+		},
+		{
+			name:  "unmodified_partial_path_no_extension",
+			input: "Makefile",
+			want:  "Makefile",
+		},
+		{
+			name:  "unmodified_partial_path_extension",
+			input: "Makefile.am",
+			want:  "Makefile.am",
+		},
+	}
+	for _, tst := range tests {
+		t.Run(tst.name, func(t *testing.T) {
+			got := normalizeFilepath(tst.input)
+			if diff := cmp.Diff(got, tst.want); diff != "" {
+				t.Fatalf("mismatch (-want +got):\n%s", diff)
+			}
+		})
+	}
+}