Your comments

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.
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.
Well, Textastic tells the system that all the programming files like .html, .cpp etc. are plain text files. Those editors claim that they can open plain text files. That's why they appear in the list.

There are two types of PDF files: binary and plain text. Both have the same extension. Plain text pdf files can be edited in Textastic. I should probably not register pdf files as plain text files though - I will change that in an update.
No, there is no official API from Apple for shortcuts like that.

Built-in shortcuts are explained in the manual.