Add progressive JPG MIME-type to default variable content types

[minimalweb]

Summary

I want to suggest including the MIME-type for progressive JPGs (image/pjpeg) in the list of variable content types. It is not an uncommon MIME-type and including it in the default list might save a bunch of people some debugging time, when something like this does not work out of the box:

<% if asset.attachment.image? %>
  <%= image_tag(asset.attachment.variant(resize: '100x100') %>
<% end %>

I know, you can configure this on a per-app basis, but I think it would also be reasonable to have this in the default list.

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[gmcgibbon]

I think if we already include variants of jpeg by default (image/jpg), and browsers support progressive jpegs, we should too. Can you please add a changelog entry?

[minimalweb]

Done.

[gmcgibbon]

👍 Thanks! Please squash your commits and we can look at getting this merged.

[scivola]

I think the non-standard MIME type image/pjpeg has nothing to do with whether the JPEG is progressive or not.

Could you see “Uploaded MIME Types” in the following official page?
MIME Type Detection in Windows Internet Explorer

The old Internet Explorer provides image/pjpeg for an uploading JPEG file, even if it is not progressive.

So, the following sentence in the CHANGELOG is not so suitable, I think.

Add progressive JPG to default list of variable content types