ASL files are style (layer) files for the image editing software Adobe Photoshop. Asl files contain pre-made styles or effects that can be applied to layers or objects of a Photoshop document. A style can include shadow options, borders, or textures and patterns.
To apply styles, the Styles window must be open. Here you can use the small arrow in the upper right corner to select the menu item 'Load styles'. Usually, however, ASL files are stored under 'Preferences / Styles' in the Photoshop program folder. In this way they are also directly accessible via the Stilmen.