[FA-misc] Fixes file caching

FictionArchive.Service.FileService/Controllers/S3ProxyController.cs

@@ -36,7 +36,15 @@ namespace FictionArchive.Service.FileService.Controllers
                     Key = decodedPath
                 });
                 
-                return new FileStreamResult(s3Response.ResponseStream, s3Response.Headers.ContentType);
+                Response.Headers.CacheControl = "public, max-age=604800"; // 7 days
+                Response.Headers.LastModified = s3Response.LastModified?.ToString("R");
+
+                if (!string.IsNullOrEmpty(s3Response.ETag))
+                {
+                    Response.Headers.ETag = s3Response.ETag;
+                }
+
+                return File(s3Response.ResponseStream, s3Response.Headers.ContentType);
             }
             catch (AmazonS3Exception e)
             {