feat: Implement file upload and folder creation features by GO (#13903)

### What problem does this PR solve?

feat: Implement file upload and folder creation features

- Add file upload route in router.go
- Add file operation methods in dao/file.go
- Add util/file.go for file type detection and filename handling
- Implement file upload and folder creation endpoints in handler/file.go
- Implement file upload and folder creation logic in service/file.go
- Modify response message format in memory.go
- Add document count method in dao/document.go

### Type of change

- [x] New Feature (non-breaking change which adds functionality)

internal/util/file.go

@@ -0,0 +1,134 @@
+//
+//  Copyright 2026 The InfiniFlow Authors. All Rights Reserved.
+//
+//  Licensed under the Apache License, Version 2.0 (the "License");
+//  you may not use this file except in compliance with the License.
+//  You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+//  Unless required by applicable law or agreed to in writing, software
+//  distributed under the License is distributed on an "AS IS" BASIS,
+//  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+//  See the License for the specific language governing permissions and
+//  limitations under the License.
+//
+
+package util
+
+import (
+	"path/filepath"
+	"regexp"
+	"strings"
+)
+
+const (
+	FileTypePDF    = "pdf"
+	FileTypeDOC    = "doc"
+	FileTypeVISUAL = "visual"
+	FileTypeAURAL  = "aural"
+	FileTypeFOLDER = "folder"
+	FileTypeOTHER  = "other"
+)
+
+var (
+	filenameLenLimit = 255
+)
+
+func init() {
+}
+
+func normalizeFilename(filename string) (string, bool) {
+	if filename == "" {
+		return "", false
+	}
+	base := filepath.Base(filename)
+	base = strings.TrimSpace(base)
+	if base == "" || len(base) > filenameLenLimit {
+		return "", false
+	}
+	return strings.ToLower(base), true
+}
+
+func FilenameType(filename string) string {
+	normalized, ok := normalizeFilename(filename)
+	if !ok {
+		return FileTypeOTHER
+	}
+
+	if matched, _ := regexp.MatchString(`.*\.pdf$`, normalized); matched {
+		return FileTypePDF
+	}
+
+	docExtensions := []string{
+		"msg", "eml", "doc", "docx", "ppt", "pptx", "yml", "xml", "htm", "json", "jsonl", "ldjson",
+		"csv", "txt", "ini", "xls", "xlsx", "wps", "rtf", "hlp", "pages", "numbers", "key",
+		"md", "mdx", "py", "js", "java", "c", "cpp", "h", "php", "go", "ts", "sh", "cs", "kt",
+		"html", "sql", "epub",
+	}
+	for _, ext := range docExtensions {
+		if strings.HasSuffix(normalized, "."+ext) {
+			return FileTypeDOC
+		}
+	}
+
+	audioExtensions := []string{
+		"wav", "flac", "ape", "alac", "wv", "mp3", "aac", "ogg", "vorbis", "opus",
+	}
+	for _, ext := range audioExtensions {
+		if strings.HasSuffix(normalized, "."+ext) {
+			return FileTypeAURAL
+		}
+	}
+
+	visualExtensions := []string{
+		"jpg", "jpeg", "png", "tif", "gif", "pcx", "tga", "exif", "fpx", "svg", "psd", "cdr",
+		"pcd", "dxf", "ufo", "eps", "ai", "raw", "WMF", "webp", "avif", "apng", "icon", "ico",
+		"mpg", "mpeg", "avi", "rm", "rmvb", "mov", "wmv", "asf", "dat", "asx", "wvx", "mpe",
+		"mpa", "mp4", "mkv",
+	}
+	for _, ext := range visualExtensions {
+		if strings.HasSuffix(normalized, "."+ext) {
+			return FileTypeVISUAL
+		}
+	}
+
+	return FileTypeOTHER
+}
+
+func SanitizeFilename(filename string) string {
+	if filename == "" {
+		return ""
+	}
+	filename = strings.TrimSpace(filename)
+	if filename == "" {
+		return ""
+	}
+
+	filename = strings.ReplaceAll(filename, "\\", "/")
+	filename = strings.Trim(filename, "/")
+
+	parts := strings.Split(filename, "/")
+	var sanitizedParts []string
+	for _, part := range parts {
+		if part != "" && part != "." && part != ".." {
+			sanitizedParts = append(sanitizedParts, part)
+		}
+	}
+
+	unsafeRegex := regexp.MustCompile(`[^A-Za-z0-9_\-/]`)
+	for i, part := range sanitizedParts {
+		sanitizedParts[i] = unsafeRegex.ReplaceAllString(part, "")
+	}
+
+	result := strings.Join(sanitizedParts, "/")
+	return result
+}
+
+func GetFileExtension(filename string) string {
+	ext := filepath.Ext(filename)
+	if len(ext) > 0 && ext[0] == '.' {
+		return strings.ToLower(ext[1:])
+	}
+	return strings.ToLower(ext)
+}