Fix bug with POST uploads returning 400 client error

This fixes a bug I was seeing where moto wouldn't accept a POST upload from my client software. For whatever reason the file came in `request.files` rather than `request.form`. 

In case you're curious, here's the request that needed this change:

    POST / HTTP/1.1
    Host: xxxxx.s3.amazonaws.com
    Content-Type: multipart/form-data; boundary=Boundary+832DF3B4960B5C10
    Accept: */*
    Expect: 100-continue
    User-Agent: Xxxxx/1.0 (Mac OS X Version 10.9.2 (Build 13C64))
    Accept-Language: en-GB;q=1, en;q=0.9, fr;q=0.8, de;q=0.7, zh-Hans;q=0.6, zh-Hant;q=0.5
    Accept-Encoding: gzip, deflate
    X-Forwarded-For: 127.0.0.1
    X-Forwarded-Host: xxxxx.s3.amazonaws.com
    X-Forwarded-Server: s3.amazonaws.com
    Connection: Keep-Alive
    Content-Length: 1022

    --Boundary+832DF3B4960B5C10
    Content-Disposition: form-data; name="AWSAccessKeyId"

    xxxxxxxxxxxxxxxxxxxx
    --Boundary+832DF3B4960B5C10
    Content-Disposition: form-data; name="key"

    116b8311bee6ee747c7b7e7f527e8c2f.raw
    --Boundary+832DF3B4960B5C10
    Content-Disposition: form-data; name="policy"

    <snip>
    --Boundary+832DF3B4960B5C10
    Content-Disposition: form-data; name="signature"

    <snip>
    --Boundary+832DF3B4960B5C10
    Content-Disposition: form-data; name="file"; filename="116b8311bee6ee747c7b7e7f527e8c2f.raw"
    Content-Type: application/octet-stream

    <snip>
    --Boundary+832DF3B4960B5C10--