No and yes!

During "Save As" the created extension is defined by UltraEdit by the file type below the name field. If you select *.txt, it will be .txt. The current file type setting is always stored in the INI file of UltraEdit. So if you select once .txt and never change it, .txt will be used always for new file.

You can modify the list of file types at Advanced - Configuration - File Types. Bring file type .txt to top of the list. Of course, if you delete all file types except *.txt, new files will be always saved as .txt. But this "solution" is not very good, because opening files with different file extension then cannot be done easily anymore with the standard file open dialog.

I tested again what I have written with UE v12.20a+4 and the Save As dialog still uses by default the last used file extension. UltraEdit saves the last used file extension in uedit32.ini with

[Settings]
Save Filter=3

Value 3 means 3rd entry in the drop down list. In your case after moving up *.txt to top of the file extensions list this setting should have the value 1.

It's important for changing the file extension (= this setting) that you first select it from the extension list and then enter the file name WITHOUT dot and extension (name only) and then save the new file. Next time the default in the extensions list should be the extension you used before.

2rsd
Advanced->Configuration->File Types, move *.txt to the top of the list, press OK.