does anyone know a little test tool (like Poster / RestTool for Firefox) that is able to upload a file and send a text body within the same post request (Multipart)?
It is not a firefox-addon, but what I can really recommend is to use curl tool. It fits perfect when playing around with RESTful HTTP APIs because it is very close to HTTP protocol. Because it is CLI based it is more flexible as graphical addon (e.g. you can mail around or can document your api with sample calls).
E.g. doing a multipart request with curl would be:
# with '-v' verbose-switch you see some headers
# with '-F' you are "activating" single multiparts
# with '@' you are referencing file
curl -v -F [email protected] -F [email protected] http://host.com/your/multipart/endpoint
# if server needs it you can also pass Content-Type with single files
... -F "[email protected];type=text/plain" ...
What kind of multipart do you expect on server-side (e.g. multipart/form-data or multipart/mixed).
Is there a reason why it has to be a firefox addon? I have seen people using RestClient, but I never saw it working with multipart.