I do not see the same behavior, on my MacBook runinng 10.10.2

If I type a name with no extention in Save As:, I see no unselected extension, and there is no change as I vary the File Type.

If I add .aiff in the Save As: box when that is the selected export type, then that part of the name becomes unselected, and changes appropriately, which changes of file type.

If I cancel, reopen the export dialog, type a name, choose "Other Uncompressed Files" (and no extension is filled in the box) and export -- then I get a .aiff file.

So, I see no bug.

I'm seeing this bug with other formats also (Debian Xfce)

1) Import an Ogg file, say "test.ogg"
2) Export (16-bit WAV selected by default).
Audacity offers to export as "test.aiff"

(In reply to Steve Daulton from comment #2)
> I'm seeing this bug with other formats also (Debian Xfce)
> 1) Import an Ogg file, say "test.ogg"
> 2) Export (16-bit WAV selected by default).
> Audacity offers to export as "test.aiff

That's the separate bug 1315.

This affects Windows too - so I broadened the scope (assumed All)

The underlying problem would appear to be the when using "Other uncompressed files" as the "Save as type" in the Export dialog when the user goes to the "Format Options" part of the dialog and chooses something other then the default or last-used in the "Header" dropdown - Audacity does not update the file extension in the "File name" data box.

Pressing the OK button will however cause Audacity to export the file in the required format and with the correct extension  that the user specified in the "Header" dropdown.

A subsequent visit to The Export Audio dialog will show that Audacity remembers the "Header" type correctly AND it sets the extension in the "File name" correctly.  BUT changing the "Header" type to something else shows that then the "File name" extension is once again not visually changed.

This is (or could be) very confusing for the user have the displayed file-type (extension) being different to that which is normally exported - accordingly I shall upgrade this from P3 to P2.

---------------------------------------------------

Note that initial use behavior varies between Windows and Mac - on Windows the initial use default for "Other compressed files" is WAV - whereas on Mac it is AIFF.

This will need to be considered if and when we implement this Wiki proposal: https://wiki.audacityteam.org/wiki/Proposal_WAV_exports_to_have_Format_Options

I updated the Steps To Reproduce and The Release Note

Forum poster ppw1 wrote:

Well, since I'm here, I may as well write up a list of correct header-extension matches for future reference:

    AIFF (Apple/SGI) - .aiff
    AU (Sun/NeXT) - .au
    AVR (Audio Visual Research) - .avr
    CAF (Apple Core Audio Format) - .caf
    HTK (Hidden Markov Model Toolkit) - .htk
    IFF (Amiga Interchange File Format/SVX8/SV16) - .iff
    MAT4 (Octave 2.0) - .mat
    MAT5 (Octave 2.1) - .mat
    MPC (Akai MPC 2k) - .mpc
    PAF (Ensoniq PARIS) - .paf
    PVF (Portable Voice Format) - .pvf
    RAW (header-less) - .raw
    RF64 (RIFF 64) - .rf64
    SD2 (Sound Designer II) - .sd2
    SDS (Midi Sample Dump Standard) - .sds
    SF (Berkeley/IRCAM/CARL) - .sf
    VOC (Creative Labs) - .voc
    W64 (SoundFoundry WAVE 64) - .w64
    WAV (Microsoft) - .wav
    WAV (NIST Sphere) - .wav
    WAVEX (Microsoft) - .wav
    WVE (Psion Series 3) - .wve
    XI (FastTracker 2) - .xi

After some further research, it's a limitation of wxWidgets, that you cannot get at the wxFileDialog filename to change it, at least not from wxWidgets.  

I have added some code that spots that the extension has changed, but there is no way to hook that in to changing the filename.
https://github.com/JamesCrook/audacity/commit/cf442759b41466b67ba73b7924b6da4f3ef13302

So what could we do?  
1. Wait for wxWidgets to provide a way to change the filename programmatically  OR
2. Re-Write the wxFileDialog ourselves (not going to happen). OR
3. Rejig our 'Other uncompressed files' Exporter so that all its header options are separately in the FileDialog Type drop down, and 'Other Uncompressed files' disappears as a separate option.  That's a major rejig and beyond the scope of this bug.

So all in all that makes this a WONTFIX.

Fixed in:
https://github.com/audacity/audacity/commit/38bf51a

I'll be posting some testing guidance on -quality...

(In reply to Leland Lucius from comment #8)
Testing on W10 and macOS 10.15.3 with Audacity 2.4.0 df32c04

The bug as described in the title and steps appears to be fixed properly
1) when you change the header the extension on the "Save as" filename changes accordingly
2) the file is exported in the right format and with the right extension


>I'll be posting some testing guidance on -quality...

Duly noted and I shall be doing some further "testing around" - I would invite others to do likewise as Leland has been doing some serious under-the-hood work with Exports.

On the basis that this works on Widows and Mac - and the fix was provided by Leland) who works on Linux) I am going to trust Leland's fix (I trust his thoroughness in testing before releasing his fix) I am going to close it as RESOLVED.

If it later turns out not to bood on Linux we can always reopen this a Linux-oly or open a new Linux-only bug.