A good solution is the tool Yummy FTP. You are able to configure the Yummy FTP to work with Textasticapp. You have to configure the yummy ftp if you want to edit a file it should open it with Textasticapp.
1. Connect to the server with yummy ftp
2. Press the button to edit the file.
3. Textasticapp start and open the file you want to edit
4. change the file and press save. Yummy ftp detects this and upload the file to the server.

I found a Solution to use it. You are able to use it with WebDAV. :-)