Applescript: Trim text front and back example

trim_string_sample.applescript

set whichFile to choose file with multiple selections allowed
repeat with aFile in whichFile
    tell application "Finder"
        set filename to name of aFile
        set name of aFile to ((characters 4 thru -1 of filename) as string) --trim first 3
        --set name of whichFile to ((characters 1 thru -4 of filename) as string) --trim last 3
    end tell
end repeat

How would you add a string at the end of the name and before the extension?

set name of aFile to ((characters 4 thru -1 of filename) & "text" as string)

However, this adds the "text" string after the extension. Thanks.