Implement filesystem media backend

This backend stores uploaded media into files on the local filesystem.
This commit also adds a `BackendType` enum, which can be used to distinguish different media backends.

Signed-off-by: David Mehren <git@herrmehren.de>

src/media/backends/backend-type.enum.ts

@@ -0,0 +1,6 @@
+export enum BackendType {
+  FILEYSTEM = 'filesystem',
+  S3 = 's3',
+  IMGUR = 'imgur',
+  AZURE = 'azure',
+}

src/media/backends/filesystem-backend.ts

@@ -0,0 +1,26 @@
+import { MediaBackend } from '../media-backend.interface';
+import { BackendData } from '../media-upload.entity';
+import { promises as fs } from 'fs';
+import { join } from 'path';
+
+export class FilesystemBackend implements MediaBackend {
+  async saveFile(
+    buffer: Buffer,
+    fileName: string,
+  ): Promise<[string, BackendData]> {
+    // TODO: Get uploads directory from config
+    const uploadDirectory = './uploads';
+    // TODO: Add server address to url
+    const filePath = join(uploadDirectory, fileName);
+    await fs.writeFile(filePath, buffer, null);
+    return ['/' + filePath, null];
+  }
+
+  deleteFile(fileName: string, backendData: BackendData): Promise<void> {
+    return Promise.resolve(undefined);
+  }
+
+  getFileURL(fileNam: string, backendData: BackendData): Promise<string> {
+    return Promise.resolve('');
+  }
+}