Textastic can load TextMate-compatible syntax definitions - this is described in the manual.

Unfortunately, there's no API for that. See http://feedback.textasticapp.com/topic/20131-better-hardware-keyboard-support/

Sounds like an interesting idea. I'll try it out when I have some time.

You should already be able to see them. For FTP connections, you need to enable the "Show hidden files" option in the FTP connection's settings. You can edit the connection's settings by tapping on the blue arrow on the right side of the connection.

I'm considering adding Multimarkdown preview support in the future.

Can you please have a look at the server log file? It should have an error message.

Thanks!

I added png and jpg files because people wanted to be able to open them in Textastic so they can upload it to a server. Obviously they shouldn't be registered as plain text files. I'll change that.

Just use the remote preview feature. You can enter an arbitrary url there.