Allow adding attachment to AttachmentCollection as asynchronous operation

[ultimaweapon]

Is your feature request related to a problem? Please describe.
I want to add attachments to AttachmentCollection from Stream in a non-blocking fashion.

Describe the solution you'd like
Add two methods with the following signature:

Task<MimeEntity> AddAsync(string fileName, Stream stream, ContentType contentType, CancellationToken cancellationToken = default)

Task<MimeEntity> AddAsync(string fileName, Stream stream, CancellationToken cancellationToken = default)

Describe alternatives you've considered
Create a MemoryStream and copy the content of attachment to it with Stream.CopyToAsync then pass it to AttachmentCollection.Add.

Additional context
My application need to read attachments from a network socket. If I pass Stream directly to AttachmentCollection.Add it will cause thread blocking too long due to latency of the network socket; thus degrade the performance significantly.

[jstedfast]

Good suggestion. Thanks!

This will be included in the next release. In the meantime, you can use the nightly builds from https://www.myget.org/feed/mimekit/package/nuget/MimeKit

It should have a build version >= 2.11.0.26 once it finishes building.