Since I am also a user of Serif legacy and Affinity products, as well as Magix, this is quite an easy question to answer!
When you installed Photoplus you were asked whether you wanted to make the app your default for opening various image file formats, e.g. .jpg, ,bmp etc. Those Magix .HDP files are actually image files containing display data like the waveform of an audio file. Because they are image files, Photoplus and Windows "think" they are associated with that program, rather than your Magix program(s).
This is no cause for any concern: they will open with your Magix program whenever it is started and their "image" data is required. Perhaps slightly counter-intuitively, if you double-click directly on one of those files, Windows will attempt to open the image in Photoplus; the program will open but you will see an error message something to the effect "Photoplus cannot open this image file".
HTH
Jeff Forum Moderator
Edited to add: just to clarify, these files are NOT "Third-party" as per your thread title, they are very much Magix files, created by your Magix program.
Thanks Jeff. This was my first attempt to join a discussion group, as I was mystified by the odd files appearing in the MAGIX folders. I'm still new to video editing but I'm getting to grips with Easy Video and have now created a movie of some camcorder clips from 2011, with many holiday items to process. It's a god job I'm retired !
I'll just ignore the Serif files then. (Just out of interest, could they be deleted ?)
I'll just ignore the Serif files then. (Just out of interest, could they be deleted ?)
Yes, you can but it does mean that if you ever re-open the project in your Magix app (?Video Easy) the app will need to recreate those files so that the Timeline display can show a graphical representation of your complete project.
Also, just to emphasise, they are not Serif files, they are Magix files.