How to get file path of user selected .icns file in applescript?
Use {"com.apple.icns"} as the file type.

set Icon to (POSIX path of (choose file of type {"com.apple.icns"} with prompt "Select a .icns file."))




(Found by running "mdls myiconfile.icns")

kMDItemContentType             = "com.apple.icns"