Add support for GIF banners with validation and improved error messages

- Add custom validator for banner file extensions (.jpg, .jpeg, .gif)
- Update HandleTranAgreed to dynamically detect banner type from file extension
- Export FileTypeFromFilename function for use across packages
- Add comprehensive test coverage for banner validation and type detection
- Improve error messages to clearly communicate allowed file extensions

4 files changed, 166 insertions, 1 deletions

diff --git a/internal/mobius/config.go b/internal/mobius/config.go
index d04b14d..c798e54 100644
--- a/internal/mobius/config.go
+++ b/internal/mobius/config.go
@@ -7,6 +7,7 @@ import (
 	"gopkg.in/yaml.v3"
 	"os"
 	"path/filepath"
+	"strings"
 )
 
 var ConfigSearchOrder = []string{
@@ -28,7 +29,25 @@ func LoadConfig(path string) (*hotline.Config, error) {
 	}
 
 	validate := validator.New()
+	if err = validate.RegisterValidation("bannerext", func(fl validator.FieldLevel) bool {
+		filename := fl.Field().String()
+		if filename == "" {
+			return true // Allow empty since BannerFile is optional
+		}
+		ext := strings.ToLower(filepath.Ext(filename))
+		return ext == ".jpg" || ext == ".jpeg" || ext == ".gif"
+	}); err != nil {
+		return nil, fmt.Errorf("register validation: %v", err)
+	}
 	if err = validate.Struct(config); err != nil {
+		// Check if this is a BannerFile validation error and provide a better message
+		if validationErrs, ok := err.(validator.ValidationErrors); ok {
+			for _, fieldErr := range validationErrs {
+				if fieldErr.Field() == "BannerFile" && fieldErr.Tag() == "bannerext" {
+					return nil, fmt.Errorf("BannerFile must have a .jpg, .jpeg, or .gif extension (got: %s)", config.BannerFile)
+				}
+			}
+		}
 		return nil, fmt.Errorf("validate config: %v", err)
 	}
 
diff --git a/internal/mobius/config_test.go b/internal/mobius/config_test.go
new file mode 100644
index 0000000..b76b16d
--- /dev/null
+++ b/internal/mobius/config_test.go
@@ -0,0 +1,95 @@
+package mobius
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+func TestLoadConfig_InvalidBannerFileExtension(t *testing.T) {
+	// Create a temporary directory for test files
+	tmpDir, err := os.MkdirTemp("", "mobius-config-test")
+	if err != nil {
+		t.Fatalf("Failed to create temp dir: %v", err)
+	}
+	defer os.RemoveAll(tmpDir)
+
+	// Create a test config file with an invalid banner file extension
+	configContent := `
+Name: "Test Server"
+Description: "Test Description"
+BannerFile: "banner.png"
+FileRoot: "files"
+`
+	configPath := filepath.Join(tmpDir, "config.yaml")
+	if err := os.WriteFile(configPath, []byte(configContent), 0644); err != nil {
+		t.Fatalf("Failed to write config file: %v", err)
+	}
+
+	// Attempt to load the config
+	_, err = LoadConfig(configPath)
+
+	// Verify that we get the improved error message
+	if err == nil {
+		t.Fatal("Expected error for invalid banner file extension, got nil")
+	}
+
+	expectedMsg := "BannerFile must have a .jpg, .jpeg, or .gif extension (got: banner.png)"
+	if !strings.Contains(err.Error(), expectedMsg) {
+		t.Errorf("Expected error message to contain %q, got: %v", expectedMsg, err)
+	}
+}
+
+func TestLoadConfig_ValidBannerFileExtensions(t *testing.T) {
+	tests := []struct {
+		name       string
+		bannerFile string
+	}{
+		{"jpg extension", "banner.jpg"},
+		{"jpeg extension", "banner.jpeg"},
+		{"gif extension", "banner.gif"},
+		{"empty banner", ""},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			// Create a temporary directory for test files
+			tmpDir, err := os.MkdirTemp("", "mobius-config-test")
+			if err != nil {
+				t.Fatalf("Failed to create temp dir: %v", err)
+			}
+			defer os.RemoveAll(tmpDir)
+
+			// Create files subdirectory
+			filesDir := filepath.Join(tmpDir, "files")
+			if err := os.Mkdir(filesDir, 0755); err != nil {
+				t.Fatalf("Failed to create files dir: %v", err)
+			}
+
+			// Create a test config file with a valid banner file extension
+			configContent := `
+Name: "Test Server"
+Description: "Test Description"
+BannerFile: "` + tt.bannerFile + `"
+FileRoot: "files"
+`
+			configPath := filepath.Join(tmpDir, "config.yaml")
+			if err := os.WriteFile(configPath, []byte(configContent), 0644); err != nil {
+				t.Fatalf("Failed to write config file: %v", err)
+			}
+
+			// Attempt to load the config
+			config, err := LoadConfig(configPath)
+
+			// Verify that we don't get a validation error
+			if err != nil {
+				t.Errorf("Expected no error for %s, got: %v", tt.bannerFile, err)
+			}
+
+			if config.BannerFile != tt.bannerFile {
+				t.Errorf("Expected BannerFile to be %q, got %q", tt.bannerFile, config.BannerFile)
+			}
+		})
+	}
+}
diff --git a/internal/mobius/transaction_handlers.go b/internal/mobius/transaction_handlers.go
index c5b1bbb..52ea29a 100644
--- a/internal/mobius/transaction_handlers.go
+++ b/internal/mobius/transaction_handlers.go
@@ -1095,7 +1095,8 @@ func HandleTranAgreed(cc *hotline.ClientConn, t *hotline.Transaction) (res []hot
 	res = append(res, trans...)
 
 	if cc.Server.Config.BannerFile != "" {
-		res = append(res, hotline.NewTransaction(hotline.TranServerBanner, cc.ID, hotline.NewField(hotline.FieldBannerType, []byte("JPEG"))))
+		bannerType := hotline.FileTypeFromFilename(cc.Server.Config.BannerFile).TypeCode
+		res = append(res, hotline.NewTransaction(hotline.TranServerBanner, cc.ID, hotline.NewField(hotline.FieldBannerType, []byte(bannerType))))
 	}
 
 	res = append(res, cc.NewReply(t))
diff --git a/internal/mobius/transaction_handlers_test.go b/internal/mobius/transaction_handlers_test.go
index 0d975a7..1c3fac0 100644
--- a/internal/mobius/transaction_handlers_test.go
+++ b/internal/mobius/transaction_handlers_test.go
@@ -2873,6 +2873,56 @@ func TestHandleTranAgreed(t *testing.T) {
 				},
 			},
 		},
+		{
+			name: "with gif banner",
+			args: args{
+				cc: &hotline.ClientConn{
+					Account: &hotline.Account{
+						Access: func() hotline.AccessBitmap {
+							var bits hotline.AccessBitmap
+							bits.Set(hotline.AccessDisconUser)
+							bits.Set(hotline.AccessAnyName)
+							return bits
+						}()},
+					Icon:    []byte{0, 1},
+					Flags:   [2]byte{0, 1},
+					Version: []byte{0, 1},
+					ID:      [2]byte{0, 1},
+					Logger:  NewTestLogger(),
+					Server: &hotline.Server{
+						Config: hotline.Config{
+							BannerFile: "Banner.gif",
+						},
+						ClientMgr: func() *hotline.MockClientMgr {
+							m := hotline.MockClientMgr{}
+							m.On("List").Return([]*hotline.ClientConn{},
+							)
+							return &m
+						}(),
+					},
+				},
+				t: hotline.NewTransaction(
+					hotline.TranAgreed, [2]byte{},
+					hotline.NewField(hotline.FieldUserName, []byte("username")),
+					hotline.NewField(hotline.FieldUserIconID, []byte{0, 1}),
+					hotline.NewField(hotline.FieldOptions, []byte{0, 0}),
+				),
+			},
+			wantRes: []hotline.Transaction{
+				{
+					ClientID: [2]byte{0, 1},
+					Type:     [2]byte{0, 0x7a},
+					Fields: []hotline.Field{
+						hotline.NewField(hotline.FieldBannerType, []byte("GIFf")),
+					},
+				},
+				{
+					ClientID: [2]byte{0, 1},
+					IsReply:  0x01,
+					Fields:   []hotline.Field{},
+				},
+			},
+		},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {