For those who might view this topic from a search, I suspect that the problem is that the original poster is dealing with a 1 item list, {alias “data:HDRI_werkmap:workflow_ontwerp:test:1:DSC_0026.psd”}
The curly braces indicate that it’s a list.
The “trick” is to reference it as “item 1” of that list.
eg
set thefiles to (choose file with multiple selections allowed)
-- result {alias "data:HDRI_werkmap:workflow_ontwerp:test:1:DSC_0026.psd"}
set myfile to item 1 of thefiles
-- result alias "data:HDRI_werkmap:workflow_ontwerp:test:1:DSC_0026.psd"