Actually, it should do that already. Are you using the latest version (2.0)?

Do you have "Hide Extension" disabled? If it is enabled, the extension isn't displayed in the save panel.

Yes, it does.

You can actually just drag the icon from Textastic's window title bar directly into a Terminal window to insert the path to the file.

You need to reference files with relative paths, not absolute paths. Something like this shouldn't cause any problems:

<a href="otherfile.html">Other File</a>

Update: I can reproduce the issue, but it only happens when you have more than one tab open. I'll see what I can do to fix this.

I can't reproduce the issue. On my machine, windows are maximized when I click on the Dock icon of Textastic with that option enabled.

Which OS X version are you using? I'm on 10.8.5.

Also, I'm not aware that I'm doing anything that should prevent the default behavior from working.

I do plan to include a dark keyboard option when I redesign the app for iOS 7.