I've added a combined undo/redo button instead (segmented control) in Textastic 3.0.

You can add your own TextMate-compatible bundles and themes in Textastic 3.0.

New syntax highlighter with Monokai theme and the symbol list:

It's currently in beta and I'm soon going to send it to Apple for review.

In Textastic 3.0 you'll be able to use custom syntax definitions. There's a TextMate bundle available that you'll be able to use.

Since you write that you would buy the app if it supported this: Did you already try it out with Textastic or do you assume that it doesn't work?

Do you have any example code that doesn't currently work in Textastic? That would make it much easier for me to support this.

So it looks like your mail client uses the wrong encoding when displaying the file.

How did you try to open the file on the receiver's end? I can't reproduce the problem.

The file is just attached as is to the email. You need to make sure that the text editor on your computer uses utf-8 when opening the file. For example, with TextEdit on Mac OS X, you need to manually select utf-8 in the file open dialog because it doesn't detect the encoding automatically.