Removing the last 10 characters of a file name while preserving the extension
You are asking two different questions here. One is "How do I remove the last 10 characters of a file name" while the other is going from [RF] Happy Hour 1 [jdhf2345].mp4 → Happy Hour 1.mp4. 

I'm not really sure which one to answer here so I guess I'll answer the first one??

"How do I remove the last 10 characters of a file name"

set myFiles to choose file with multiple selections allowed
repeat with myFile in myFiles
    tell application "System Events"
        set myName to the characters 1 thru ((offset of "." in (name of myFile as text)) - 1) of (name of myFile as text)
        tell application "Finder"
            set myExtention to name extension of (myFile as alias)
            set myNewName to characters 1 thru (((length of myName) - 10) as number) of (myName as text)
            set name of file (myFile as text) to (myNewName &amp; "." &amp; myExtention as text)
        end tell
    end tell
end repeat