JPGs vs JPEGs in Cover Art

So I'm just in 2012 really starting to use a cell phone, and I've put some music onto a class 4 MicroSD card to listen to, but so far I can not get the phone (Sanyo Innuendo) to display album art. I know it can, because some files I haven't yet formatted to my standardized tag system show up fine. However, any one of my files that I've formatted don't work. I've been wracking my brain trying to figure out what it might be. I know the phone is picky enough that it will only display specifically encoded metadata. I thought perhaps the JPG was progressively saved or something like that, but when I extract and remove the album art, and then insert the same album art, I don't get any art (for the same file which worked before I messed with it).

The only difference I can see in the files, is MP3tag displays my (nonworking) files' albumart as: ID3v2.3image/jpeg
Whereas the mp3s - which do work - have cover mimetypes of ID3v2.3image/jpg

jpeg vs jpg

I don't know if it would be the solution to my problem, but is MP3Tag writing my album art jpgs as jpegs? Is there a way to make the jpegs show up as jpgs, whatever that would involve behind the curtain?

Examples:

Russian Choir file does not show any album art in phone, the other does.

PS: On a hunch, I used PNG album art instead of jpg/jpeg, and it works! But I cannot batch process hundreds of images right now, and even if I had my usual software, I don't want to use PNGs in my music.

It seems Mp3tag will only write image/jpeg (which is also recommended by the standard).

Is there an 'advanced mode' or anything like that to override this?

No. I guess you need to find another tool for this job.

Read this thread: mp3 player not reading cover art