Turned out pretty cool, me thinks, and while it's quite minimal, me also thinks.. it's quite powerful. I was using Bulk Rename Utility, which is free for personal use, but my own simple app is more compact, offers me everything I need for some fairly advanced renaming, and best of all, has a Dark Theme! ^ ^
So how does it work. Pretty self explanatory. Type or Paste a Path [sorry, no fancy browser yet], and choose an Extension. By default it searches for txt files, change that to anything else you need. Add or remove a prefix, add or remove a suffix, and optionally you can also replace anything with anything else, like replacing all underscores with spaces.
If you have a name clash, meaning two identical names, the app renames only the first file and then stops. It will stop on other illegal activities like using illegal characters, which in Windows you cannot use in file and folder names, like <>:"/\|?*. It stops on illegal names like AUX or COM1 and a bunch of others.. Kinda fun to know. You can name a file or folder AUX1, but you cannot name it AUX. Microsoft rules.
I might improve it in the future, it would be nice if it had a small TreeView or Browser, as opposed to only a Text input. The Extension thing is not ideal. It could remember settings, at least the last Path used. There is no functionality for automatic numbering. And ideally, there could be an Undo. Atm it's up to you to be wise and not rename things madly, because once you click Save, your files will be renamed, period, no Undo.
Tested under Windows 10 x64. Microscopic 45 kb download. Has a fixed UI size of 1000 x 635 pixels, no scaling sorry. WinForms! ^ ^ Windows will probably tell you it's an unknown app, just click More Info and Run Anyway [feel free to scan with antivirus, but also.. ya know, my website is not some shady hub of cyber-criminality]. If you use it and enjoy it, or not enjoy it, or find bugs, or have any feedback about it, lemme know. ^ ^
Free to use for everyone everywhere forever and ever. Updated to Version 1.1 : ]
Download here and go rename some files!