I am currently using the Chrome Addon Postman - REST Client to easily create POST / GET request.
Now I want to debug an upload script and I am looking for a way to make requests encoded as "multipart/form-data" so that I can send also a file.
UPDATE: I have created a video on sending multipart/form-data requests to explain this better.
Actually, Postman can do this. Here is a screenshot
Newer version : Screenshot captured from postman chrome extension
Another version
Older version
Make sure you check the comment from @maxkoryukov
Be careful with explicit Content-Type header. Better - do not set it's value, the Postman is smart enough to fill this header for you. BUT, if you want to set the Content-Type: multipart/form-data - do not forget about boundary field.