Hello @LyricsLover,
I did the test, it doesn't work, there must be something I'm not doing correctly
File: Rename_JPG_Extension_in_lowercase.cmd
@ECHO OFF
REM JPG extensions in lowercase, you can use a Windows-CMD-Batchfile like this and rename all the .JPG with a double click:
for %%F in (*.JPG) do ren "%%F" "%%~nF.jpg"
Which version of Windows are you trying to run this command on?
Please double-check if you have one or more .JPG filenames In the same directory as your Rename_JPG_Extension_in_lowercase.cmd
Hello @LyricsLover,
Sorry, I tested it with an Mp3tag tool for .JPG files, and it didn't work.
It works. I tried again as you described, in a TEST folder with 3 audio files and the file Rename_JPG_Extension_in_lowercase.cmd.
When I double-click on the file:
Rename_JPG_Extension_in_lowercase.cmd
My two image files, .JPG, change to .jpg.
It works. 
Thank you. 
I would have liked to be able to do this with an Mp3tag tool with the audio files I select. I don't think this programming is possible...
It should be possible.
It's almost midnight here in Europe. I will check it tomorrow and let you know.
You can try it with this modified version and use it as new Mp3tag Tool.
Adjust the Path and Batch filename to your own local version.
As Parameter use
"%_folderpath%"
REM The passed "%_folderpath%" from Mp3tag already contains a trailing Backslash
REM Example: C:\Temp\mp3\Album\
REM Therefore no divider is necessary between %1 and *.JPG
REM The ~ character between % and 1 removes the passed double quotes at start and end of the %_folderpath% content
for %%F in ("%~1*.JPG") do ren "%%F" "%%~nF.jpg"
PAUSE
You can delete the last PAUSE command. This is only useful for identifying errors that occur during execution.
This Windows CMD Batch renames ALL *.JPG to *.jpg in the same folderpath of the selected File(s). As it renames ALL *.JPG stored in the same directory, it is only necessary to call it once per directory and not for every file in the same directory.
Please let me know if this works as Mp3tag Tool for you.
Hello @LyricsLover, thank you very much, your programming works perfectly with Mp3tag Tools. 
In the Mp3tag Actions command, I programmed 3 different folders to store the audio file covers.
I imagine it's possible to add to the program:
Rename_JPG_Extension_to_lowercase.cmd
The same programming, for the following folders:
C:\BSI32\Album Art Web Server
C:\BSI32\Album Art\
Mp3tag Actions Command
Export audio covers to the Folder, you are currently working on.
It works perfectly with your programming.
Export audio covers to BSI32, Album Art Web Server Folder (Simian)
C:\BSI32\Album Art Web Server\
Export audio covers to BSI32, Album Art Folder (Simian) [Adjusts audio file covers to 200x200]
C:\BSI32\Album Art\
The above Batch CMD file receives the content of %_folderpath%.
The %_folderpath% variable will be filled per track where you right click and choose Tools -> "Rename JPG Extension to lowercase".
No matter what folder you pass from Mp3tag to the Tool, this folder path will be used to adjust *.JPG to *.jpg in it.
Example:
If you you right click on a track in
C:\BSI32\Album Art Web Server\
you pass
C:\BSI32\Album Art Web Server\
as content of %_folderpath%
and the Tool will rename all *.JPG to *.jpg in this specific path.
You don't have to change the Batch CMD itself for different folders.
@LyricsLover, I completely understand what you just wrote to me, and it's a very good way to proceed....
When I click on my three audio files with images in Mp3tag and run the Actions command:
"Export audio covers to the Folder, you are currently working on"
and a Tools .JPG to .jpg
I don't open the folder where the three songs I selected are located, because I'm sure Mp3tag automatically exports the three images to the folder where the three audio files are located. I'll open the folder later to check the images and make sure all my image files are .jpg, which works wonderfully...
I think it's possible to add to this file so that this .JPG to .jpg command can write the data to all three folders at the same time
"Rename_JPG_Extension_to_lowercase.cmd"
"Export audio covers to the folder you are currently working on"
C:\BSI32\Album Art Web Server\
C:\BSI32\Album Art\
I noticed that if the image files aren't in the folder, the "Rename_JPG_Extension_to_lowercase.cmd" program doesn't report any errors...
Later, when I open my three folders, all my image files will be in .jpg format.
Sorry, this is not possible with a variable like %_folderpath%
You can only pass 1 variable content %_folderpath% from Mp3tag to the batch CMD file at a time.
BUT:
If you like, you can try to adjust the Batch CMD yourself and copy & paste the same line for your fixed pathnames like
for %%F in ("C:\BSI32\Album Art Web Server\*.JPG") do ren "%%F" "%%~nF.jpg"
and
for %%F in ("C:\BSI32\Album Art\*.JPG") do ren "%%F" "%%~nF.jpg"
This would adjust all *.JPG to *.jpg once in the given %_folderpath% and in addition all *.JPG in your two added fixed folders.
If the Batch CMD does not find any *.JPG files in the specified folder, then it just does nothing.
I don't think that a warning as "nothing to do..." is very helpful.
But feel free to customise the Batch CMD to suit your needs.
For example you could try to include tests if any JPG files have been found and providing useful text output.
A huge thank you @LyricsLover,
Your program works very well:
Export audio covers to the Folder, you are currently working on
C:\BSI32\Album Art Web Server
For
C:\BSI32\Album Art
I wouldn't need it
Adjusts audio file covers to 200x200
I hadn't thought of this detail, when Mp3tag Adjust all covers, the 3 images are programmed as standard. image/jpeg
See my image
I'm taking this opportunity to ask @Florian, if he agrees, to add a formula in Actions (Quick) and Actions of Mp3tag, to convert .JPG to .jpg This command already exists when the images are resized...
You're right, a warning wouldn't be very useful when the program can't find any data.
I'm definitely going to customize your little program to my liking.
Again A big thank you LyricsLover for your help
