Get-JiraIssueAttachmentFile: Set Accept header based on Mime time of attachment

[wisemoth]

Description

The attachment objects have a MimeType property. This should be used to set the Accept header in the request to download the actual attachment; however the existing code refers to (non-existing) MediaType property on the Powershell object, thereby setting the Accept header value to null.

Motivation and Context

A partial fix for apparently "successful" download of file attachments. Without this change the pipeline output is True for each attachment downloaded, but the attachment did not download. This PR doesn't change the fautly error handling, but implements a trivial underlying fix to constructing the request to fetch the download.
I believe that the error handling code needs to be fixed too, but I've not had a chance to do that.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project.
• I have added Pester Tests that describe what my changes should do.
• I have updated the documentation accordingly.