Implement File API methods

- GetAllTaskFiles: retrieve all files attached to a task
- CreateTaskFile: upload file with base64 encoding
- DownloadTaskFile: download file with base64 decoding
- RemoveTaskFile: delete a task file
- TaskScope.GetFiles and UploadFile for fluent API
- Automatic base64 encoding/decoding for file content
- Comprehensive test coverage

files.go

@@ -0,0 +1,77 @@
+package kanboard
+
+import (
+	"context"
+	"encoding/base64"
+	"fmt"
+)
+
+// GetAllTaskFiles returns all files attached to a task.
+func (c *Client) GetAllTaskFiles(ctx context.Context, taskID int) ([]TaskFile, error) {
+	params := map[string]int{"task_id": taskID}
+
+	var result []TaskFile
+	if err := c.call(ctx, "getAllTaskFiles", params, &result); err != nil {
+		return nil, fmt.Errorf("getAllTaskFiles: %w", err)
+	}
+
+	return result, nil
+}
+
+// CreateTaskFile uploads a file to a task.
+// The file content is automatically base64 encoded.
+// Returns the ID of the created file.
+func (c *Client) CreateTaskFile(ctx context.Context, projectID, taskID int, filename string, content []byte) (int, error) {
+	params := map[string]any{
+		"project_id": projectID,
+		"task_id":    taskID,
+		"filename":   filename,
+		"blob":       base64.StdEncoding.EncodeToString(content),
+	}
+
+	var result int
+	if err := c.call(ctx, "createTaskFile", params, &result); err != nil {
+		return 0, fmt.Errorf("createTaskFile: %w", err)
+	}
+
+	if result == 0 {
+		return 0, fmt.Errorf("createTaskFile: failed to upload file")
+	}
+
+	return result, nil
+}
+
+// DownloadTaskFile downloads a file's content by its ID.
+// The content is returned as raw bytes (decoded from base64).
+func (c *Client) DownloadTaskFile(ctx context.Context, fileID int) ([]byte, error) {
+	params := map[string]int{"file_id": fileID}
+
+	var result string
+	if err := c.call(ctx, "downloadTaskFile", params, &result); err != nil {
+		return nil, fmt.Errorf("downloadTaskFile: %w", err)
+	}
+
+	// Decode base64 content
+	content, err := base64.StdEncoding.DecodeString(result)
+	if err != nil {
+		return nil, fmt.Errorf("downloadTaskFile: failed to decode content: %w", err)
+	}
+
+	return content, nil
+}
+
+// RemoveTaskFile deletes a file from a task.
+func (c *Client) RemoveTaskFile(ctx context.Context, fileID int) error {
+	params := map[string]int{"file_id": fileID}
+
+	var success bool
+	if err := c.call(ctx, "removeTaskFile", params, &success); err != nil {
+		return fmt.Errorf("removeTaskFile: %w", err)
+	}
+
+	if !success {
+		return fmt.Errorf("removeTaskFile: delete failed")
+	}
+
+	return nil
+}