How do I POST a JSON file as MessageBody for Amazon SQS?

[Henrik]

I run ElasticMQ locally to emulate Amazon SQS, and I want to send a JSON file as a MessageBody. Here's an example request that works:

curl 'http://localhost:9324/queue/foo?Action=SendMessage&MessageBody={"action":"hey"}'

If I want to send a large JSON file, it would make more sense to to that as a POST, something like this:

# Doesn't work:

curl 'http://localhost:9324/queue/foo?Action=SendMessage' -X POST \

     -H "Content-Type: application/json" --data @./bigdata.json

Is there a way to make this work?