OData $orderBy expression. Comma-separated list of fields with optional 'asc'/'desc'.
Example: 'updatedAt desc, fileName asc'
Available fields (files): categoryId, createdAt, createdBy, deliverablesCount, fileId, fileName, fileTypeId, updatedAt, sessionsCount
Special case for folders: unsupported file-based filters are ignored, and folder sorting is limited to: folderId, folderName, parentFolderId, createdBy, updatedAt, createdAt. Unknown fields are dropped.