This has been requested by a few people now. While it's not that hard to add the column, it could be tricky to present to the user in an intuitive way.
Here are a few options I've considered, maybe you can let me know your thoughts:
A simple checkbox allowing the user to tick it if they want to include the file line number with the standard output rows. The column would be hard coded to something like "__dfs_filelinenumber" (as in something unlikely to already be used)
As above except add a text box allowing the user to specify the column name.
Somehow integrate it into the Advance Column Editor page of the UI. This could cause confusion with regards to ordering it with the actual file columns.
Option 1 would definitely be the easiest. Not sure Option 3 would even make sense.