How to get DzAsset via Filename?

How can one get a DzAsset object from a filename (prefered absolute but can be relative)?
According to another post it should work with AssetMgr but I haven't manged to figure out how. 
I have tried "getProductsForFiles", "findProductsForFile" and "findAssetsForFile" but they always return a empty array for booth absolute and relative filenames.

 

var oAssetMgr=App.getAssetMgr();
var list=[];
list.push(sFilename);
//var oAssets=oAssetMgr.getProductsForFiles(list);
var oAssets=oAssetMgr.findProductsForFile(list);
//var oAssets = oAssetMgr.findAssetsForFile( list, false, true );
print(oAssets.length);
//print(oAssets.products.length);