re: but Thunderbird overrides this with its own default settings for opening attachments. If you already have an entry for jpg files then it is because you set it up the first time you chose to open a jpg file.

re : Options>attachments>jpeg>'select helper application' the Photos app does not appear in the list and I do not know how to browse to it as it is not a conventional app in the programs folder

From the 'Action' drop down list you would select 'Use Other'. a new window opens allowing you to choose from a more extensive list; you may need to use the scroll bar to see everything..

If the Photo app does not appear in the list then you have to click on 'Browse' button to locate the .exe file manually.

but do not know where the windows Photo app is located - now that is not really a Thunderbird issue :)

I do not use Win10, but I've had a look on google and located this info which you could use to try to locate it: c:\program files\windowsapps\microsoft.windows.photos_15.1001.16470.0_x64__8wekyb3d8bbwe\microsoft.photos.exe;

You may discover this could be a hidden file, so you would need to make hidden files and folders visible first.

Thanks for the response. I accept that my question was arguably more a Windows 10 issue than a Thunderbird issue but it only seems to arise in TBird and I hoped that another user would have had an identical problem and solved it. Your solution via 'Windowsapps' unfortunately does not work as clicking on the .exe file gives an error dialogue shown in the attached png.

I now seem to have solved the issue. One of the options in the 'Select helper application' popup is the delphic word 'TWINUI'. This amazingly refers to the windows 10 shell and chooses the system default application for the file type. Only in Microsoft...

helper application

re :I now seem to have solved the issue. One of the options in the 'Select helper application' popup is the delphic word 'TWINUI'. This amazingly refers to the windows 10 shell and chooses the system default application for the file type. Only in Microsoft...

Thanks for the feedback info. As I'm still on 'Vista', TWINUI would not be in my list, but it is very useful to know that selecting TWINUI has now allowed thunderbird image attachments to open as expected using the Photo.app.

A point to clarify...Just in case people wonder what is 'Select helper application' : It is the window you see when you select 'Use other'.