Add a -ContentType parameter to Set-PodeResponseAttachment

[Badgerati]

At the moment, the Set-PodeResponseAttachment function assumes the content type of the response based on the attachment/file's extension (ie: .json gets mapped to application/json).

In most cases the extensive mapping is correct. However, there should be a new -ContentType parameter on the Set-PodeResponseAttachment function that, if supplied, will be used instead of the mapping.

This new parameter should be an optional string parameter, with the help comment updated. If supplied, the $WebEvent.Response.ContentType should be set as this value instead of:

$WebEvent.Response.ContentType = (Get-PodeContentType -Extension $ext)

[Badgerati]

Completed by #388