Displaying the contents of a file of indeterminiate length

I am working on the UX for an application that takes in an excel file and displays results of its processing to the user.

In this application, to begin file processing, it needs to know where the header row is located. So the user is asked to define a header row, I have lobbied to make it as automated as possible to make it a good experience, and only ask for the user to define it if this processing is wrong.

One of the main problems for displaying it to the user, as you can imagine, is these files could be tens of thousands of lines long, or only a few lines. Then to complicate matters a little more, the header is not always at the top of the file either, because there's no standard way of putting the file together, and our users would not respond well to standardization. So potentially the header row could be (at most, from our experience) 10 rows down, or possibly 20 rows down in a really weird case.

So how would I display the contents of a user file, for header selection, and afford the use case where it may not be in the first 12 lines shown? I have a partial solution, but I'm not happy with it (see attached). In this solution the user would simply click a row. So if you're able to give visual examples that would be great, if not, any standards, conventions, resources and things of that sort would be helpful too.