fix of attaching multiple json and text attachments to the same step

[rjktcby]

This fixes having multiple attachments for a single step (if they are not all plain texts)

I also added mime type check for 'application/json' because it is officially recomended way of attaching jsons from cucumber-js, as desribed here

I made step.text an array because it is the most obvious way to have attachments of different types (plain text and json) displayed together.

Added test json that was used to reproduce the problem as
test/unit/data/custom-metadata-json/multiple_different_attachements.json

[coveralls]

Coverage increased (+0.01%) to 98.754% when pulling e9137c8 on Habidatum:master into 4119a01 on wswebcreation:master.

[wswebcreation]

Tnx for the PR, it looks good!