Add ability to specify the uploaded file content-type

[leplatrem]

If unset, let the server assign it as it does now.

If set the attachment passed to kinto.js's collection.createRecord(..) should contain the type

kinto-admin/src/components/record/RecordCreate.tsx

Lines 37 to 41 in c648566

	({ __attachment__: attachment, ...record }) => {
	const {
	params: { bid, cid },
	} = match;
	createRecord(bid, cid, record, attachment);

• kinto.js supports extracting the type from the data URL