Content-Type for files uploaded via S3 automatically set to application/xml

### Describe the bug

When I upload a file to S3 (using a multipart upload request) the content-type of the file will be application/xml unless I specify otherwise. This seems incorrect as a content-type should be omitted if unknown or, at worst, default to application/octet-stream. Per RFC 7231 (3.1.1.5):

> A sender that generates a message containing a payload body SHOULD
> generate a Content-Type header field in that message unless the
> intended media type of the enclosed representation is unknown to the
> sender. If a Content-Type header field is not present, the recipient
> MAY either assume a media type of "application/octet-stream"
> ([RFC2046], Section 4.5.1) or examine the data to determine its type.

This ended up causing a bit of confusion here (https://github.com/apache/arrow/issues/11934). An S3 client was trying to be intelligent and inspect the XML data if the file was an XML file and this issue caused the client to inspect files it shouldn't.

### Expected behavior

If the content type of a file is not set then the file should either have no content-type or the content-type should be set to application/octet-stream.

### Current behavior

The file's content-type is set to application/xml

### Steps to Reproduce

Reproducible Gist: https://gist.github.com/westonpace/9c3a0baa48083f33aa4880c0cb6a602b

### Possible Solution

When the user does not specify a content-type either leave it unset or default to application/octet-stream

### AWS CPP SDK version used

1.8.185

### Compiler and Version used

GCC 9.3.0

### Operating System and version

Ubuntu 20.04.3