Fix empty filename when Content-Disposition header is missing in /dl command

Add filenameFromURL helper to extract filename from URL path when
Content-Disposition header is empty or not present. This fixes the issue
where direct link downloads fail due to empty filename.

Co-authored-by: krau <71133316+krau@users.noreply.github.com>

core/tasks/directlinks/execute.go

@@ -45,10 +45,14 @@ func (t *Task) Execute(ctx context.Context) error {
 			fetchedTotalBytes.Add(resp.ContentLength)
 			file.Size = resp.ContentLength
 			if name := resp.Header.Get("Content-Disposition"); name != "" {
-				// Set file name
+				// Set file name from Content-Disposition header
 				filename := parseFilename(name)
 				file.Name = filename
 			}
+			// Fallback: extract filename from URL if Content-Disposition is empty
+			if file.Name == "" {
+				file.Name = filenameFromURL(file.URL)
+			}
 
 			return nil
 		})

core/tasks/directlinks/util.go

@@ -173,6 +173,35 @@ func parseFilenameFallback(cd string) string {
 	return decodeFilenameParam(value)
 }
 
+// filenameFromURL extracts filename from a URL path.
+// It uses the last path segment and removes any query parameters.
+// Returns empty string if the URL cannot be parsed or has no valid path.
+func filenameFromURL(rawURL string) string {
+	u, err := url.Parse(rawURL)
+	if err != nil {
+		return ""
+	}
+
+	// Get the path and extract the base name
+	path := u.Path
+	if path == "" || path == "/" {
+		return ""
+	}
+
+	// Find the last path segment
+	idx := strings.LastIndex(path, "/")
+	if idx >= 0 && idx < len(path)-1 {
+		filename := path[idx+1:]
+		// URL decode the filename
+		if decoded, err := url.QueryUnescape(filename); err == nil {
+			return decoded
+		}
+		return filename
+	}
+
+	return ""
+}
+
 var progressUpdatesLevels = []struct {
 	size        int64 // 文件大小阈值
 	stepPercent int   // 每多少 % 更新一次

core/tasks/directlinks/util_test.go

@@ -0,0 +1,78 @@
+package directlinks
+
+import (
+	"testing"
+)
+
+func TestFilenameFromURL(t *testing.T) {
+	tests := []struct {
+		name     string
+		url      string
+		expected string
+	}{
+		{
+			name:     "simple file",
+			url:      "https://example.com/file.zip",
+			expected: "file.zip",
+		},
+		{
+			name:     "file with path",
+			url:      "https://example.com/path/to/document.pdf",
+			expected: "document.pdf",
+		},
+		{
+			name:     "url with query params",
+			url:      "https://example.com/file.mp4?token=abc123",
+			expected: "file.mp4",
+		},
+		{
+			name:     "url with fragment",
+			url:      "https://example.com/file.txt#section",
+			expected: "file.txt",
+		},
+		{
+			name:     "url encoded filename",
+			url:      "https://example.com/%E6%B5%8B%E8%AF%95.zip",
+			expected: "测试.zip",
+		},
+		{
+			name:     "url encoded Chinese filename",
+			url:      "https://example.com/10%E6%9C%8817%E6%97%A5(6).mp4",
+			expected: "10月17日(6).mp4",
+		},
+		{
+			name:     "root path only",
+			url:      "https://example.com/",
+			expected: "",
+		},
+		{
+			name:     "no path",
+			url:      "https://example.com",
+			expected: "",
+		},
+		{
+			name:     "empty url",
+			url:      "",
+			expected: "",
+		},
+		{
+			name:     "file with spaces encoded",
+			url:      "https://example.com/my%20file.txt",
+			expected: "my file.txt",
+		},
+		{
+			name:     "complex path with multiple slashes",
+			url:      "https://cdn.example.com/a/b/c/d/e/video.mkv",
+			expected: "video.mkv",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			result := filenameFromURL(tt.url)
+			if result != tt.expected {
+				t.Errorf("filenameFromURL(%q) = %q, expected %q", tt.url, result, tt.expected)
+			}
+		})
+	}
+}