Security Message when trying to open JPG files in Windows Explorer

Firstly apologies if I have posted this query in the wrong section.
I am running Windows 7 Home Premium and totally out of the blue I am now faced with a Security Warning every time I try to open a JPG file from within Windows Explorer. Having tried a number of experiments I have noted the following:

• only files that I have downloaded from the internet appear to be affected
• files that I have had on my hard drive for quite some time do not trigger the warning but only the more recent ones
• the warning message does not show if I open the JPG from within my graphics program (I'm using Paint Shop Pro)

I cannot think of a reason for this to have suddenly started, although last night I downloaded the Windows 10 free upgrade but havn't yet run the install routine. Surely that would not be a factor though.

Having searched Google it would seem the only fix for this annoying message is a fairly complicated routine involving changing settings in the registry, which I am a bit worried about doing. Obviously I could simply untick the "Always ask before opening this file" box but I would need to do this with every other affected file individually. Is there no way this could be done 'en-mass'?

Sorry, but I have just come across an anomaly that may be a factor in my problem detailed above. I tried to save a test JPG from the internet and have noticed that the default file type is "Micrografx Picture Publisher Image Object". I do have an old version of Micrografx on my PC but seldom use it. How that became the default file type is a mystery. It would appear that JPGs saved as this file type are the ones causing the security warning. Can anyone advise how I go about changing that default file type? I have clicked on the arrow next to it but the only other option is "All files".