Detect whether clipboard item is image data using AppleScript

Having to write and maintain an osascript command as you have is really not the best way to go in this use case IMO.

Using a Run AppleScript action, instead of a Run Shell Script action, the following example AppleScript code will resolve the issue you are currently having, and makes the code much easier to read and edit:

on run {input, parameters}
    
    set cbInfoAsString to (clipboard info) as string
    
    if cbInfoAsString does not contain "«class furl»" and ¬
        cbInfoAsString contains "TIFF picture" then
        
        set formattedDate to do shell script ¬
            "date -j '+%Y-%m-%d at %I.%M.%S %p'"
        
        tell application "Finder" to set thePath to ¬
            (insertion location as alias) & ¬
            formattedDate & ".png" as string
        
        do shell script "/usr/local/bin/pngpaste " & ¬
            the quoted form of the POSIX path of thePath
        
    else if cbInfoAsString contains "«class furl»" then
        
        tell application "System Events" to ¬
            keystroke "v" using control down
        
    end if
    
end run

Notes:

• POSIX path is a part of Standard Additions not Finder, and should not be wrapped within a tell statement of Finder.
• If not setting the date with the date command, use the -j option.
• I modified your date command to use 12-Hour Time as typically 24-Hour Time does not use AM/PM and have added made additional modifications to make it more readable, and in line with the system default used with screen shots.
• If you want 24-Hour Time then use e.g.: "date -j '+%Y-%m-%d at %H.%M.%S'"
• As coded, if there is anything other than a file(s)/folder(s) or just an image, the script will not attempt to process text if that's what's on the clipboard.