I don’t think sending an actually file is the best way to do this but we can think of something that works better over HTTP perhaps.
Right now you can use PlantText to do what you ask, but you have to use an HTTP GET request that looks like this for example…
The last part of the URL is the “deflated” PlantUML markup for the sample diagram seen below…
Bob -> Alice : hello
So, this would work for you but you would need to programatically deflate the PlantUML to a string of text and send it to the URL, using whatever language or software you are familiar with. Deflating code is not really simple and may not exist readily with the tools you are using. If it does then just go for it.
I could simplify this though by removing the deflating step for you. I could create a URL that you could send an HTTP POST request to, with the PlantUML markup for your diagram in plain text (not deflated), and return to you the diagram image in the response. You would still need to know how to programatically send a POST request with the PlantUML markup text in the body of the request.
If you are not a programmer, then this is probably too much for you to do on your side. This would be pretty simple for me to do my part on the server side. I will plan on doing it if you think it would be helpful.
It may take a little time as I am currently working on multiple document support in PlantText first. Just let me know.