[3.29] Cannot upload zip file because mimetype becomes a promise

[wesley-at-hl]

When I upload a zip file in 3.29, the filestack picker thinks it is not in list of accepted file types, even though .zip is included.

I traced it to getMimetype is sometimes returning a promise which is then compared as if it is a value

[mkeemon]

We're also encountering issues with mimetype, but we're seeing that it's always being coerced to text/plain regardless of the file's extension. Downgrading to 3.28.0 was the best temporary solution for us @wesley-at-hl.