Improve FileProxy Handling

When proxying an asset file from Amazon S3 or Google Storage,
we previously ignored important headers such as

   - Content-Type
   - Content-Length
   - Cache-Control

We also ignored the storage service's HTTP response,
effectively assuming 200, and just blindly passed on
the content body.  In the case of any errors or redirects,
we would interpret that (empty or meaningless) body as
the asset itself.

Instead, we now proxy those HTTP headers and treat
any non-200 as an error.

apps/meteor/app/file-upload/server/lib/FileUpload.ts

@@ -562,7 +562,32 @@ export const FileUpload = {
 	) {
 		res.setHeader('Content-Disposition', `${forceDownload ? 'attachment' : 'inline'}; filename="${encodeURI(fileName)}"`);
 
-		request.get(fileUrl, (fileRes) => fileRes.pipe(res));
+		request.get(fileUrl, (fileRes) => {
+			if (fileRes.statusCode !== 200) {
+				res.setHeader('x-rc-proxyfile-status', String(fileRes.statusCode));
+				res.setHeader('content-length', 0);
+				res.writeHead(500);
+				res.end();
+				return;
+			}
+
+			// eslint-disable-next-line prettier/prettier
+			const headersToProxy = [
+				'age',
+				'cache-control',
+				'content-length',
+				'content-type',
+				'date',
+				'expired',
+				'last-modified',
+			];
+
+			headersToProxy.forEach((header) => {
+				fileRes.headers[header] && res.setHeader(header, String(fileRes.headers[header]));
+			});
+
+			fileRes.pipe(res);
+		});
 	},
 
 	generateJWTToFileUrls({ rid, userId, fileId }: { rid: string; userId: string; fileId: string }) {