fix: Clear error message when provided config is not a text file (#11787)

config/config.go

@@ -581,7 +581,17 @@ func LoadConfigFile(config string) ([]byte, error) {
 	}
 
 	// If it isn't a https scheme, try it as a file
-	return os.ReadFile(config)
+	buffer, err := os.ReadFile(config)
+	if err != nil {
+		return nil, err
+	}
+
+	mimeType := http.DetectContentType(buffer)
+	if !strings.Contains(mimeType, "text/plain") {
+		return nil, fmt.Errorf("provided config is not a TOML file: %s", config)
+	}
+
+	return buffer, nil
 }
 
 func fetchConfig(u *url.URL) ([]byte, error) {

config/config_test.go

@@ -1,10 +1,13 @@
 package config
 
 import (
+	"bytes"
 	"fmt"
 	"net/http"
 	"net/http/httptest"
 	"os"
+	"os/exec"
+	"path/filepath"
 	"reflect"
 	"runtime"
 	"strings"
@@ -26,6 +29,32 @@ import (
 	"github.com/influxdata/telegraf/plugins/parsers/json"
 )
 
+func TestReadBinaryFile(t *testing.T) {
+	// Create a temporary binary file using the Telegraf tool custom_builder to pass as a config
+	wd, err := os.Getwd()
+	require.NoError(t, err)
+	t.Cleanup(func() {
+		err := os.Chdir(wd)
+		require.NoError(t, err)
+	})
+
+	err = os.Chdir("../")
+	require.NoError(t, err)
+	tmpdir := t.TempDir()
+	binaryFile := filepath.Join(tmpdir, "custom_builder")
+	cmd := exec.Command("go", "build", "-o", binaryFile, "./tools/custom_builder")
+	var outb, errb bytes.Buffer
+	cmd.Stdout = &outb
+	cmd.Stderr = &errb
+	err = cmd.Run()
+
+	require.NoError(t, err, fmt.Sprintf("stdout: %s, stderr: %s", outb.String(), errb.String()))
+	c := NewConfig()
+	err = c.LoadConfig(binaryFile)
+	require.Error(t, err)
+	require.ErrorContains(t, err, "provided config is not a TOML file")
+}
+
 func TestConfig_LoadSingleInputWithEnvVars(t *testing.T) {
 	c := NewConfig()
 	require.NoError(t, os.Setenv("MY_TEST_SERVER", "192.168.1.1"))