DATAMONGO-2021 - Use getObjectId() instead of getFilename() for opening the GridFS download stream.

Using the file name leads to duplicate resource streams as file names are not unique therefore we're using the file's ObjectId to lookup the file content.

Original pull request: #581.

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/gridfs/GridFsTemplate.java

@@ -240,7 +240,7 @@ public class GridFsTemplate implements GridFsOperations, ResourcePatternResolver
 
 		Assert.notNull(file, "GridFSFile must not be null!");
 
-		return new GridFsResource(file, getGridFs().openDownloadStream(file.getFilename()));
+		return new GridFsResource(file, getGridFs().openDownloadStream(file.getObjectId()));
 	}
 
 	/*