[39831] introduced more strict MIME type checking for uploads, which
resulted in unintetionally blocking several filetypes that were
previously valid. This change uses a more targeted approach to MIME
validation to restore previous behavior for most types.
Props blobfolio, iandunn, ipstenu, markoheijnen, xknown, joemcgill.
Merges [40124] and [40125] to the 4.7 branch.
Fixes#39550, #39552.
git-svn-id: https://develop.svn.wordpress.org/branches/4.7@40134 602fd350-edb4-49c9-b593-d223f7449a82
Joe McGill