In Win 10, I have default programs defined by filetype, so .jpg files are opened in Irfanview, .pdf in Adobe Reader etc. Selecting a file in Windows Explorer and double clicking it or opening it automatically launches the file with those default programs.
In eMClient, I have default attachment actions of open defined in settings for both jpg and pdf. However, when I double click or open the attachment in a mail, I get the Win 10 pop up asking me ‘How do you want to open this file?’ and offering me the default of my chosen app, but listing other possible programs, and asking me to tick a box if I want to make one option permanent. But whatever I do, the next time I open an attachment, the same thing happens. As the default is already defined, why is this happening and how can I stop it so that eMClient simply opens the attachment with the default program defined?
I am using the same version of Windows but an unreleased version of eM Client. That might be it, though I don’t recall seeing the problem in earlier versions. There is a newer version than you are using available for download at http://www.emclient.com/release-history. You might want try that and see if there is a difference.