The Misc page lets you configure various server and connection-related settings for a site entry in the FTP Address Book.
The Server Commands section has the following options:
The Transfer Mode section lets you control how files are transferred to and from this FTP site. The FTP protocol allows files to be transferred either as ASCII (with the idea that they are plain text files - the FTP server will automatically convert line endings between LF and CR/LF depending on the computers involved in the transfer), or as Binary (files are transferred as-is, with no changes to the data). Normally you would want files transferred as Binary, but you can use this option to switch to ASCII mode, or also select Automatic.
In automatic mode, Opus will switch modes automatically based on the file extension of the file being transferred. You can configure which file extensions are treated as ASCII in this mode from the Misc page for the Default Settings entry. There you can configure a list of file extensions that will be recognised as plain-text files and transferred in ASCII mode.