"file-manager" is not one of the standard icon names defined by FDO icon naming specification[1]. That causes problem when using icon themes which do not provide that non-standard icon(one example is Oxygen, but there are many others )

So I would suggest using another FDO standard icon instead, like "system-file-manager".

[1] http://standards.freedesktop.org/icon-naming-spec/icon-naming-spec-latest.html#names.