You might be able to use GetToken(), and NumTokens()
For example, let's say you're storing the file contents in a global named file_contents, i.e., Global: Set 'file_contents' to File.LoadFileToString(AppPath & "file.txt")
Well, NumTokens(global('file_contents'),NewLine) should tell you how many lines there are in the file_contents global.
So, you could possibly do this:
For "loopname" from NumTokens(global('file_contents'),NewLine) - 10 to NumTokens(global('file_contents'),NewLine):
with the action: System: Set 'somevar' to global('somevar') & GetToken(global('file_contents'),LoopIndex,NewLine)
And set your text box to show global('somevar')
Or you could clear the text box's text first, and instead have your action set the text box's text to: Editbox.Text & GetToken(global('file_contents'),LoopIndex,NewLine) ..if it's an editbox.
I hope I made enough sense. I'm tired.