Menu app icon > Options > Attachments > Incoming What is set as Action for opening jpg ? Is it 'Use TWINUI (default)' ? If no then choose that option.

Or Delete the action and try to open jpg and choose 'Use TWINUI (default)' .

What's your version of TB (Help/About TB)? A fix for opening attachments was included in 68.4.1.

Well, I've kinda got round it but it's still all a bit odd. Going to TB's Tools->Apps->Attachments I found just'default' against JPG files, which is what I'd not been able to open. The problem had suddenly appeared just a few days ago. I don't have the option to use the suggested TWINUI thingy, I am not offered it as an option nor to type in. My Win10 system defaults for JPG files is Paint which has been working normally and still works normally if I save the JPG out of the message first. I got round the immediate problem by specifying Serif Photoplus as TB wouldn't come up with Paint as an option. However Photoplus is slow to start up so far from ideal. I was on TB 68.3.1. I have now updated to 68.4.1 but I'm in the same hole as TB doesn't offer for me to go back to where I was. So I can now open attachments using Photoplus, thanks to the tips above, but remain baffled as to how all this came about in the first place, and how to get back to getting TB to use Paint as it was happily doing just a few days ago!

In the Options/Attachments/Incoming tab, click the Action for jpg content type, select Use Other, then Browse... to mspaint.exe, which is located in C:\Windows\System32

Bingo sfhowes, that's got it! Many thanks, and indeed thanks to all who responded.

.jpg won't open since the last update, which fixed .pdf to open. I followed the instructions above to go to options, but I don't have .jpg in the list for actions. How do you add .jpg and .jpeg to the list? Thank you!

ToSt said

.jpg won't open since the last update, which fixed .pdf to open. I followed the instructions above to go to options, but I don't have .jpg in the list for actions. How do you add .jpg and .jpeg to the list? Thank you!

If there's no entry for jpg, open a jpg attachment and select the opening app in the window that appears, then check the box 'Do this automatically...' to have the choice stored in Options.

http://kb.mozillazine.org/Actions_for_attachment_file_types

All sorted now, thanks for help everyone.

I have almost the same problem, but no matter what setting I use In TB for jpg files, clicking on the attachment just stores them in my appData/Local/TEMP directory, and Picasa doesn't open the file tho it updates the contents of that folder. I had specified that jpg files be opened by PIcasa Photo Viewer and all had been working well for years -- this started a few months ago. All other file types open as indicated, e.g. csv is opened by Excel, ...

I first tried all the different options for jpg file types, including save file, Use Default, Always Ask, choosing another program to open jpg's, ... and all did the same thing. I then deleted the JPG option for incoming attachments and tried again to open it, but it still just goes to the TEMP dir, and I don't get prompted for anything. Same behavior for all when using SAFE mode too.

[Note in the attached image I show the choices for JPEG, not JPG, because I just deleted the JPG action and it won't come back. Anyhow I've tried every combination, including the bottom part about where to save files to as well as the drop down list]

I'm on Win 10 x64 using TB 68.10.0 (32 bit)

How to fix this?