Show API-uploaded files along attachments

Summary:
The problem: a GET request on /files/ returns only email attachments. To change this I modified the filtering options. Instead of only querying Parts I changed the code to query Blocks and filter blocks who don't have a content type (IIRC email parts must have a content type and we force API-uploaded files to have one too).

Emfree, could you tell me what you think of this?

Also, could you tell me why we are making an api-uploaded file a block? Wouldn't it make more sense to have it be a part which could be linked to a Draft?

Fixes T298.

Test Plan: Applied the changes. Checked that a request on /files returned API-uploaded files.

Reviewers: emfree, spang

Reviewed By: spang

Maniphest Tasks: T298

Differential Revision: https://review.inboxapp.com/D334