Revert "fix the spec"

This reverts commit 143bd3e2b02503c5bc767608677fa8052804eef5.
use string mime type as lookup fails

revert when RestClient accepts the PR and releases a new version
(https://github.com/rest-client/rest-client/pull/248)
Merge pull request #15 from econsultancy/fix_csv_mime_type

Fix csv mime type