A related report:
Filename is no longer selected in Save / Save As / Open dialogs in macOS Mojave
http://illustrator.uservoice.com/forums/601447/suggestions/44505108

As for Windows — alas, it’s an OS-level behavior and Illustrator has to comply the rules.

[on macOS] when saving files only the filename is selecetd, not the extension. It used to be fine with Adobe apps until some versions back suddenly the whole file name including extension is selected in the Save as dialog.

Adding some text used to be easy. With a tap on the right arrow key the cursor moved between de file name and the extension. When I do this now the cursor is placed after the extension so I have to use the mouse or left arrow key to move the cursor 3-4 positions back. This may be OK on Windows but it is definately not Mac behaviour.

Please fix it.

Enclosed is a sample of a Save as dialog in macOS.

macOS 11.5.1 / Adobe Illustrator 25.3.1

[on macOS] when saving files only the filename is selecetd, not the extension. It used to be fine with Adobe apps until some versions back suddenly the whole file name including extension is selected in the Save as dialog.

Adding some text used to be easy. With a tap on the right arrow key the cursor moved between de file name and the extension. When I do this now the cursor is placed after the extension so I have to use the mouse or left arrow key to move the cursor 3-4 positions back. This may be OK on Windows but it is definately not Mac behaviour.

Please fix it.

Enclosed is a sample how a Save as dialog should behave in macOS.

This has changed. Now when I export my files from illustrator, the cursor is automatically at the end of the .png.
I'm never going to manually overwrite the file extension! If I pressed the right arrow in the past, the cursor stopped before the dot and I could add a bit or change the file name easily. now I have to remember to press the left arrow several times to get past the file extension.

Well, it looks normal for me :) I’m on Windows, and it’s how it always does here, at least for me.
If I see it right, when you hit Enter in the end, it assumes it’s .ai file and works fine, so it’s more a cosmetic thing. Still, I know how these things could irritate.
I suggest you contact Adobe directly to let them search the system for what my cause this.

I tried it with Illustrator 24.2.3 under OS X 10.15.6

On my colleague's Mac (OS X 10.14.6, Illustrator 24.2.1) it works like before on my Mac (highlightening only the filename without the extension; a behaviour that I know since I work with AI, approx. 12 years).

It always selects extensions for me, both for Save As and Export.
Are you on Windows or a Mac? I believe this is OS level thing.
Still,, needs investigation. Please try other windows, and also record it on video/GIF.

The selected area as of now was only the filname without the extension. That allowed you to simply give the file a new name. Now that the whole field (including the extension) is selected, you should give your new file an extension manually.
That should be corrected.

P.S.: If you export files then this „behaviour” doesn't occur.

This is SO messing up my workflow. 20+ years of using Adobe apps on a Mac, and now this change? What the heck, Adobe?!??