When renaming a file (i.e., when you double-click on a filename, or press F2, or press Enter after renaming the previous file), currently the entire filename including its extension is initially selected, much like in the normal Windows Explorer UI.
A Windows Explorer alternative called XYplorer has a UI in which the initial selection does not include the filename extension. For example, rather than SomeArtist - SomeTitle.flac being selected for editing, only SomeArtist - SomeTitle is selected; the .flac is not selected. You're still free to adjust the selection on your own, of course; only the initial selection is affected.
I found this interface was extremely convenient for mass manual renaming. I could press F2 and just start pasting and typing, hitting Enter after each new name, and I wouldn't have to adjust any selections or deal with filename extensions at all. This would make sense in Mp3tag because extensions rarely need to be changed, except perhaps for case conversion.
So, I'd like to suggest that Mp3tag do the same thing as XYplorer, or at least make it available as an option to toggle.
Thanks for your consideration, and thanks for Mp3tag!