[REMINDER] The DioptraClient support for file uploads via the post method needs testing coverage

[jkglasbrenner]

The new files and data arguments for the DioptraClient's post method that were introduced in PR #708 do not have proper tests. This issue is a reminder/check that we need to make sure that it gets proper testing coverage after we add integration tests for REST API endpoints that will accept file uploads, e.g. #575, #588.

In the meantime, the following branch https://github.com/usnistgov/dioptra/tree/test-client-uploading-support serves as a sanity check that the new file uploading support works as expected. This branch adds a simple /echoes endpoint to the REST API that allows you to upload files, and the server echoes back the name and size of the files you upload. The branch also contains a simple integration test exercising the /echoes endpoint. Both the requests-based client and the one based on Flask test client work as expected. The basic logic in that branch may be adapted for building tests in #575 and #588.