Add checksums and content type to file_core

[hannes-ucsc]

https://docs.google.com/document/d/1NsibP8g-NeLnksxlcBWQsSj5Zg_uimCDaAF1qB_qkjg/edit#heading=h.eeyjdjt197l9

TODO: fill this out

For which schema is a change/update being suggested?

e.g.: I would like to request an update to the cell_line.json schema.

What should the change/update be?

e.g.: I would like to add a new field - random_integer - to this schema to allow data contributors to assign a random integer to the cell line.

This update constitutes a <major/minor/patch> change to the schema(s) it affects.

What new field(s) need to be changed/added?

e.g.:

• Field name: random_integer
• Field description: A random integer.
• Field type: integer
• Required: no
• Examples: 42
• CV or enum: no

Why is the change requested?

e.g.: Data contributors need to be able to assign a random integer to a cell line in order to keep track of it.