Correct mime types for json subjects

[srallen]

The TESS project should be using json files as one of their subject locations, with a json file extension and a mime type of application/json. Currently libmagic is not correctly detecting these file types and staging subjects have been uploaded as txt files with a mime type of text/plain.

We can set workflow configs to load a particular subject viewer, but I still would like to do validation on the expected json structure so we don't attempt to render something that has something wrong with its data. We typically expect text file subjects to be rendered just as plain text and are typically transcription projects, not data that should be plotted.