I can't find this in the documentation, but my experience tells me this:
_TAG is a short-cut, or you might say "pseudo-field", that represents all fields in the tag when creating actions. The filename is not part of the tag so it is not included.
_All is similar, but it includes the filename (the extension is omitted).
For Case conversion action:
If Field is _FILENAME --> only the filename will be affected, but not the extension.
If Field is _TAG --> only the tag(s) will be affected, but not the filename.
If Field is _ALL --> the tag(s) and the filename will be affected, but not the extension.
If you want to control the case of the extension, you may use a separate action.
For example, to make extensions lower case:
Action: Replace with regular expression
Field: _FILENAME
Regular expression: ([^.]*)$
Replace matches with: $lower($1)
case-sensitive comparison: check-mark
FYI
Terminology
Album, albumartist, comment, title, etc. are not properly called tags, they are called fields.
The tag is all of the user written metadata.
For example: an MP3 file may have an ID3v1 tag and/or an ID3v2 tag and/or a APEv2 tag.
Each tag is made up of all of the fields used.
%_tag% is the placeholder that is populated with the names of the tag types in the file (eg. ID3v1 APEv2).
Asking for Help - some suggestions
It is highly unusual to create an action by directly creating and/or editing the mta file from outside Mp3tag.
So it is rarely helpful to post the contents of the mta file.
It's clearer for others if they can see the edit dialogue for the action.
And nobody wants to wade through an action group that has multiple actions.
It is better to isolate specific issues and deal with them one at a time.
You can post a screenshot of the edit dialogue, but it is probably better to take the time to describe it in text.
Use the same form as seen in the edit dialogue. (like I did, above)
I suggest using Preformatted text (</> in the toolbar of the reply window) for user-written information to avoid problems when copying and pasting.