This tutorial will show you how change the contents of ? blocks and brick blocks to various things that aren't available by default.
Let's start!
1. Open the jyotyu tileset.

2. Go in the Tile Behaviours tab and click on any of these tiles:

These tiles are all duplicates of other tiles in the jyotyu tileset, so there's no use in keeping them.

3. Change its tile behaviour depending on what you want it to contain:
Brick blocks:
Blue shell: 35 00 10 00
Fire flower: 34 00 10 00
Mega mushroom: 97 00 10 00
Mini mushroom: 95 00 10 00
Two coins: 45 00 10 00

? blocks:
Blue shell: 35 00 10 00
Fire flower: 34 00 10 00
Two coins: 45 00 10 00

4. Find the object in the Objects tab which uses the tile you modified. If you don't find any object that uses the tile you modified, add a new object to the list that uses it.
Do note that the icon showing the contents of the block won't change no matter what the block actually contains. Because of this, I recommend changing its description to match its content to make it easier to remember which is which.
This can be done by pressing the "Create Descriptions" button and changing the contents of the description box that will have appeared. Once you're done, save.

5. Optionally, you can turn the blue shell behaviour into a mushroom/blue shell behaviour by using the following code hack:
https://nsmbhd.net/thread/2569-misc-patches-thread/?from=540#52910

You're done! Here's the result:

_________________________
Nothing to say, so jadnjkfmnjamnfjkldnajfnjkanfjdksan jsdnvj m.

So you mean you choose "Blue Shell" in the tileset editor and it becomes a Blue Shell?

Whiiiiiiiiiii :o Amazin

I know it's really easy to do but most people might just have never thought about this so I posted it because I thought it could've been useful to some people.
_________________________
Nothing to say, so jadnjkfmnjamnfjkldnajfnjkanfjdksan jsdnvj m.

Oh. I was hoping it could spawn any shell. Damn. Still useful to a few people I guess.
_________________________
hey look, I did a thing

I know most people knows the tile behaviour. My point was that the people might not have thought about using it on a ? Block instead of a brick block.
_________________________
Nothing to say, so jadnjkfmnjamnfjkldnajfnjkanfjdksan jsdnvj m.

...Why would you know to use it on a brick block and not a ? block?
_________________________
hey look, I did a thing

It only appears with brick block selected, not ? block.
_________________________
Nothing to say, so jadnjkfmnjamnfjkldnajfnjkanfjdksan jsdnvj m.

Oh, ok.
_________________________
hey look, I did a thing

Thx for this tutorial, hidden on the second page (but not yet I'm posting)

But I have a problem: those blocks doesn't work on DeSmuMe, but it works whan I play on my DS. Does somebody knows where is the problem and a solution?