I copied a bunch of JPEG files to a 1T hard drive. After a while, I noticed that many of them had been converted to DICOM format, with the file extension being .jpg.dcm. This conversion was done auto-magically for reasons I don't know. They are all 1K in size. An online file viewer does not open them.

I value these pics.

I can recopy them from the several computers they are on, but I don't want this conversion to happen again.

WTF happened? I don't understand online sources. 'Splain it to me.

My first suspect is a plugin that takes action when copying files.

Teracopy is what you seek.

I simply used the Windows Explorer copy/paste function like I always have. The sources were my old laptops or old portable HDD. None of the sources have experienced the JPG to DCM conversion. I've been wondering if there is something on the new 1T HDD that provided me this "favor".

This is puzzling.   I've seen similar things happen in the past.  Never knew what caused it.
Thankfully, I was able to go into dos/command line and rename them.

If nothing actually made changes to the file other than the file name/ extensions you might be in luck.

If they are all in one folder or just a few folders and sub folders, could you try a batch rename? If hundreds of folders, it'd get old quick.

You could go into DOS and try something like:
ren *.* *.jpg (putting a space between *.* and *.jpg)

try just one file 1st.
ren testfilename.jpg.dcm testfilename.jpg

If it works and is still viewable, then do the batch.

I did one on my own PC a few mins ago between calls as a sorta simple test.

Example:
C:\Test>dir  (looking for a files ending in .jpg.dcm)
Volume in drive C is Windows
Volume Serial Number is F000-D700
10/14/2021  10:34 AM              ..
10/14/2021  10:34 AM                44 testfile.jpg.dcm (I created a file called testfile.jpg.dcm for this test)
              2 File(s)             88 bytes
              2 Dir(s)  170,846,584,832 bytes free
C:\Test>
C:\Test>ren testfile.jpg.dcm testfile.jpg (then I renamed it using the command showed)

C:\Test>dir
Volume in drive C is Windows
Volume Serial Number is F000-D700
Directory of C:\Test
10/14/2021  10:34 AM                44 testfile.jpg   <----(is the result)
             
I've done it before, but like I said, please test it on one file before possibly ruining a whole group.  
I'm no expert, but I've done some file manipulations in DOS and Linux a couple of times in my past.

If works, you should be able to do batches with ren *.* *.jpg (Rename every file in that dir to the same name.jpg, you might have to fine tune it a bit)

Hope that helps.  Let me know if it works or not.
 

Are the source drives external drives that may have been used by "My Book Backup" software? Here's the clue that prompted the question:

When you copied the image files, are you certain you selected the JPG files (and not "[File1].jpg.dcm")?
Attached File

Interesting. One of the source drives is made by Western Digital. The destination drive is by Seagate. I'll investigate.

This is weird af,.... I am at a loss to think of any reason any company would have a plugin or program like this or why windows would allow it to run.   Be to not delete your old backups.