I don't know if this is what you're looking for, but I set up a workflow like this:
- when I activate Alfred and type the keyword “png”, Alfred will limit my search to only png files.
- follow the keyword “png” with the space key, and then type the name of the png I want to open.
- hit Enter, and the png will be opened with Affinity Photo, because the keyword “png” is connected to the Open File with Affinity Photo as below.
If someone has a quicker workflow, please share.