Use mime sniffing to detect mimetype, fallback to file extension

[jonasfj]

We currently detect mimetype based on file extension... I propose that:

1. task.payload.artifact should have an optional contentType property
2. We should follow the mimesniff spec to detect mimetype, falling back to file-extension if that doesn't work.

Note:

• https://golang.org/pkg/net/http/#DetectContentType
• https://golang.org/pkg/mime/#TypeByExtension

Hence, minesniff is already implemented for us :)

[jonasfj]

So dustin made a good point that looking at contents might not be smart since it's less predictable..

[petemoore]

++ for adding optional artifact field - actually I was thinking the same thing, and never got round to documenting my suggestion!