there's no need to use text mode if you want the file name. It should be selectable using the grouping for Document Version / File Name. If for some reason you need to add it as text mode to an existing text mode grouping, you can just create another report, select the doc version / filename grouping, convert it to text mode, and copy/paste it into your working report (just change the numbers in the grouping to 1 instead of 0).
File name and Proof name are two different things (at least in my world). If you wanted proof name instead of file name, just take your file name text mode and everywhere it says "file", change this to "proof"