Total Commander

I use FTP or SFTP to connect to my Synology NAS

Beside Photos, I have also some movies, stored as iso files.
TC makes a complete download, then ask me "open with"

My question:
is it possible to avoid the download, just open and stream
the file to the immediately starting player?
Of course with the option to stop it anytime?

is it possible to avoid the download

No

FTP is a single client to server connection.
This means, the file is transfered to the same client, which opens the connection.

Next problem: An ISO file cannot be streamed.
It has to be downloaded completely first.

Indeed streaming isn't currently supported with FTP. If the NAS supports WebDAV, you could use that for streaming - but only streamable formats like mp4, mpg or 3gp.

well ok, streaming is not the main target.
I like to play my movies, and iso is a bit difficult...

with linux, I can mount my NAS and then can easy
access with any player as a local file... and no need
to download it this works fine...

is there any way to do the same with android, or with TC,
means: showing the file as local to an app?