Test API that receives content type 'multipart/form-data'

Good evening,

In our application we have a lambda function that receives files. The lambda is declared as follows:

```
@upload_routes.route(routes_common.DATA_UPLOAD_ROUTE,
methods=['POST'],
content_types=['multipart/form-data'],
authorizer=authorizer,
cors=True)
```

I would like to implement a unit test for this lambda, as we have done for all the others.
However, I'm not sure about what is the right way of passing the files.

With the `request` library, I would use the parameter `files`. What's the best way of testing such lambda with the Caliche Client test class?

Thanks in advance!

Start with the Chalice Client test class and the multipart/form-data route declaration shown in the issue. Determine how the client accepts uploaded files, then confirm the approach with a unit test that exercises the lambda and verifies the received file content.

Written by the indexing model from the issue text.