The getBytes  method may return random data in particular conditions.

If the chunk data lost in the server(or not saved properly), leaving only the metadata, the getData method would return random data.

Reproduce data:

```
> db.fs.files.findOne({fileName:'test.txt'});                  
{
	"_id" : ObjectId("51417e1df85ac56f1200023c"),
	"fileName" : "test.txt",
	"owner" : "kmj3njwojk",
	"permission" : {
		"other" : {
			"read" : 1,
			"write" : 0
		},
		"group" : {
			"read" : 1,
			"write" : 0
		},
		"unreg" : {
			"read" : 1,
			"write" : 0
		},
		"deny" : {
			"read" : 0,
			"write" : 0
		}
	},
	"tag" : [ ],
	"path" : {
		"fn" : "test.txt"
	},
	"filename" : "/tmpfs/phpXAPPjB",
	"uploadDate" : ISODate("2013-03-14T07:37:01.951Z"),
	"length" : 9633,
	"chunkSize" : 262144,
	"md5" : "d41d8cd98f00b204e9800998ecf8427e"
}
> db.fs.chunks.find({files_id: ObjectId("51417e1df85ac56f1200023c")});
>
```