I'm aware that this is a 12-year-old response, but I wrote an action that will CHANGE the file extension, for ANY Mp3Tag Readable input file, to your choosing. Basically, on a new Mp3Tag field (i.e. see "[#3]" from the excerpt), which you MUST create, such as %new_file_extension%, you can freely enter the extension of your choosing. However, keep in mind that, even though Mp3Tag will push your entry from %new_file_extension%, Mp3Tag will ONLY read "changed" file extensions from the same family tree (i.e. container). For example, if the original input file extension was .mp4, and you changed it to .m4v, .m4a, or .m4b, Mp3Tag will be able to read those changes and allow you to write back to .mp4 as well, if you wish. However, if you changed the .mp4 to .mkv, .m4r, .goku, etc, Mp3Tag, even though it's capable of reading an mkv file, will NOT read the newly changed extension from mp4 to mkv, and .m4r (iTunes ringtone) and .goku aren't readable via Mp3Tag. Therefore, you can just use the "undo" button (or Ctrl + Z). Basically, you can write to anything, but Mp3Tag might not be capable of reading the newly outputted extension. Likewise, it will read an .mka file extension that was previously .mkv extension. Nonetheless, I will also attach the .mta file as well.
EXPLANATION (Excerpt from .mta file):
[#0] Preserves the ORIGINAL filename by copying %_filename% to %preserve_original_filename%.
[#1] Preserves the ORIGINAL file extension by copying %_extension% to %preserve_original_extension%.
[#2] DELETES both the filename (i.e. %_filename%) and file extension (i.e. %_extension%), COMPLETELY, which was why I replaced with nothing.
[#3] Will combine "[#0]" and the NEW file extension. Moreover, I included an $if2(x,y) format string to PRESERVE the original file extension (i.e. %preserve_original_extension%) from "[#1]", just in case the %new_file_extension% Mp3Tag field was blank because if the $if2(x,y) string was absent, and the action was ran without an entry present in %new_file_extension%, the action will delete the file extension, leaving your file without a file extension.
[#4] Deletes the fields %preserve_original_filename% and %preserve_original_extension%.
[#0]
T=5
1=%_filename%
F=PRESERVE_ORIGINAL_FILENAME[#1]
T=5
1=%_extension%
F=PRESERVE_ORIGINAL_EXTENSION[#2]
T=4
F=_FILENAME
1=^(.?).(.)$
2=
3=0[#3]
T=5
1=%preserve_original_filename%.$if2(%NEW_FILE_EXTENSION%,%preserve_original_extension%)
F=_FILENAME[#4]
T=9
F=PRESERVE_ORIGINAL_FILENAME;PRESERVE_ORIGINAL_EXTENSION
UNIVERSAL File Extension CHANGER by wneclass2018 (October 12th 2022).mta (358 Bytes)