Add file upload support with body_FormData and file_Base64 functions

- Add FileCache for caching local files and remote URLs in memory
- Update body_FormData to accept variadic key-value pairs with file support
  - Use @ prefix for file paths (local or HTTP/HTTPS URLs)
  - Use @@ to escape literal @ values
- Add file_Base64 function for Base64 encoding files
- Update documentation with new syntax and examples

internal/sarin/filecache.go

@@ -0,0 +1,102 @@
+package sarin
+
+import (
+	"fmt"
+	"io"
+	"net/http"
+	"os"
+	"path"
+	"path/filepath"
+	"strings"
+	"sync"
+	"time"
+)
+
+// CachedFile holds the cached content and metadata of a file.
+type CachedFile struct {
+	Content  []byte
+	Filename string
+}
+
+type FileCache struct {
+	cache          sync.Map // map[string]*CachedFile
+	requestTimeout time.Duration
+}
+
+func NewFileCache(requestTimeout time.Duration) *FileCache {
+	return &FileCache{
+		requestTimeout: requestTimeout,
+	}
+}
+
+// GetOrLoad retrieves a file from cache or loads it using the provided source.
+// The source can be a local file path or an HTTP/HTTPS URL.
+func (fc *FileCache) GetOrLoad(source string) (*CachedFile, error) {
+	if val, ok := fc.cache.Load(source); ok {
+		return val.(*CachedFile), nil
+	}
+
+	var (
+		content  []byte
+		filename string
+		err      error
+	)
+	if strings.HasPrefix(source, "http://") || strings.HasPrefix(source, "https://") {
+		content, filename, err = fc.fetchURL(source)
+	} else {
+		content, filename, err = fc.readLocalFile(source)
+	}
+
+	if err != nil {
+		return nil, err
+	}
+
+	file := &CachedFile{Content: content, Filename: filename}
+
+	// LoadOrStore handles race condition - if another goroutine
+	// cached it first, we get theirs (no duplicate storage)
+	actual, _ := fc.cache.LoadOrStore(source, file)
+	return actual.(*CachedFile), nil
+}
+
+func (fc *FileCache) readLocalFile(filePath string) ([]byte, string, error) {
+	content, err := os.ReadFile(filePath) //nolint:gosec
+	if err != nil {
+		return nil, "", fmt.Errorf("failed to read file %s: %w", filePath, err)
+	}
+	return content, filepath.Base(filePath), nil
+}
+
+func (fc *FileCache) fetchURL(url string) ([]byte, string, error) {
+	client := &http.Client{
+		Timeout: fc.requestTimeout,
+	}
+
+	resp, err := client.Get(url)
+	if err != nil {
+		return nil, "", fmt.Errorf("failed to fetch URL %s: %w", url, err)
+	}
+	defer resp.Body.Close() //nolint:errcheck
+
+	if resp.StatusCode != http.StatusOK {
+		return nil, "", fmt.Errorf("failed to fetch URL %s: HTTP %d", url, resp.StatusCode)
+	}
+
+	content, err := io.ReadAll(resp.Body)
+	if err != nil {
+		return nil, "", fmt.Errorf("failed to read response body from %s: %w", url, err)
+	}
+
+	// Extract filename from URL path
+	filename := path.Base(url)
+	if filename == "" || filename == "/" || filename == "." {
+		filename = "downloaded_file"
+	}
+
+	// Remove query string from filename if present
+	if idx := strings.Index(filename, "?"); idx != -1 {
+		filename = filename[:idx]
+	}
+
+	return content, filename, nil
+}