manifest-list: Drop HTTP Content-Type sentence

[wking]

As Adrian Colley points out in #390, clients may not be aquiring the manifest over HTTP. In most cases, folks will know (from a Content-Type header, a descriptor media type, or other means) what type of blob they're dealing with before they look at the blob. I expect client behavior like:

If you can verify the digest, ignore Content-Type. If you can't verify the digest, respect the Content-Type and require it to match your expected media type (if any).

But in the absence of an HTTP-API spec I don't think we need to talk about this at all.

Fixes #390.

[jonboulle]

LGTM (though I'd also be happy to change this to a suggestion, e.g. "In the case of transferring images over HTTP clients COULD distinguish ..." or similar)

[wking]

On Tue, Oct 18, 2016 at 07:46:35AM -0700, Jonathan Boulle wrote:

(though I'd also be happy to change this to a suggestion, e.g. "In
the case of transferring images over HTTP clients COULD distinguish
..." or similar)

I'm fine with that or the guidelines I quote in the topic post.
Either way, this sort of advice is not manifest-list specific. We can
push it into media-types.md or the considerations.md that is in flight
with #340. Would you like me to move it to media-types.md in this PR?

[philips]

LGTM

I like @jonboulle's suggestion; lets do this in media-types.md

[wking]

On Tue, Oct 18, 2016 at 09:07:21PM -0700, Brandon Philips wrote:

I like @jonboulle's suggestion…

“COULD distinguish” isn't much guidance ;). Do you have specific
concerns with the “If you can verify…” language I quote in 1? That
quote tries to set up the following behavior tree:

1. You know the media type ahead of time.
   a. The Content-Type matches your expected media type.
   Use your expected media type to select the appropriate blob
   parser / validator.
   b. The Content-Type does not match the expected media type.
   i. You know the digest ahead of time.
   Use your expected media type to select the appropriate blob
   parser / validator. Possibly warn about the mismatch.
   ii. You don't know the digest ahead of time.
   Error out citing the mismatch.
2. You don't know the media type ahead of time.
   Use Content-Type to select the appropriate blob parser / validator.

Would you change any of those? Recommend a different tree? Just
rephrase the wording for that tree?

[jonboulle]

Do you have specific
concerns with the “If you can verify…” language I quote in [1]?

This seems fine. Please file a followup PR.