[FLINK-9769][rest] Clear FileUpload attribute after access

This closes #6270.
diff --git a/flink-runtime/src/main/java/org/apache/flink/runtime/rest/FileUploadHandler.java b/flink-runtime/src/main/java/org/apache/flink/runtime/rest/FileUploadHandler.java
index aa87cc5..28c2849 100644
--- a/flink-runtime/src/main/java/org/apache/flink/runtime/rest/FileUploadHandler.java
+++ b/flink-runtime/src/main/java/org/apache/flink/runtime/rest/FileUploadHandler.java
@@ -199,7 +199,7 @@
 	}
 
 	public static FileUploads getMultipartFileUploads(ChannelHandlerContext ctx) {
-		return Optional.ofNullable(ctx.channel().attr(UPLOADED_FILES).get())
+		return Optional.ofNullable(ctx.channel().attr(UPLOADED_FILES).getAndRemove())
 			.orElse(FileUploads.EMPTY);
 	}
 }