feat(document-service): add type-safe file extension handling with FileFormat union

- Introduced `FileFormat` union type to restrict valid document extensions.
- Added `validExtensions` array for strict extension checking in `Doc.extension` getter.
- Ensured `Doc.iconSrc` reflects only recognized file types or defaults to 'unknown'.
- Improved type safety and consistency in extension handling.

src/client/dd-hub-react/src/api/document-service.ts

@@ -1,5 +1,27 @@
 import { _documents } from "src/_mock"
 
+export type FileFormat =
+    | 'pdf'
+    | 'docx'
+    | 'xlsx'
+    | 'csv'
+    | 'pptx'
+    | 'txt'
+    | 'json'
+    | 'xml'
+    | 'html'
+    | 'jpg'
+    | 'png'
+    | 'svg'
+    | 'zip'
+    | 'md';
+
+const validExtensions: FileFormat[] = [
+    'pdf', 'docx', 'xlsx', 'csv', 'pptx',
+    'txt', 'json', 'xml', 'html', 'jpg',
+    'png', 'svg', 'zip', 'md'
+];
+
 export class Doc {
 
     static map(source?: Partial<Doc>): Doc {
@@ -27,10 +49,13 @@ export class Doc {
         return [who, when].filter(Boolean).join(separator);
     }
 
-    get extension(): string | undefined {
+    get extension(): FileFormat | undefined {
         const parts = this.name.split('.');
         if (parts.length > 1 && parts[parts.length - 1].trim() !== '') {
-            return parts[parts.length - 1].toLowerCase();
+            const ext = parts[parts.length - 1].toLowerCase();
+            
+            if (validExtensions.includes(ext as FileFormat))
+                return ext as FileFormat;
         }
         return undefined;
     }