Add MIME type to PsdImagePlugin

[samamorgan]

Fixes #4787

Changes proposed in this pull request:

• Register "image/vnd.adobe.photoshop" as PsdImageFile MIME type

[hugovk]

Please could you add tests similar to #3525?

Also, https://wordpress.stackexchange.com/a/212334 and https://www.whatisfileextension.com/psd/ suggest there's more in use:

image/vnd.adobe.photoshop

application/x-photoshop

application/photoshop

application/psd

image/psd

Should these be added too?

[samamorgan]

@radarhere Thanks for the quick check-in!

I'm not sure a mechanism exists for multiple mimetypes in this library? I'll look for examples of this in existing code and emulate that if possible. That being said, if there isn't a mechanism for this, I'd prefer we use "image/vnd.adobe.photoshop" as it's the registered IANA type (noted in issue #4787).

I'll work on tests!

[samamorgan]

Looking at all of the image plugins, I only see one mimetype registered for each, so without major restructures around how mimetypes work in this library, I think we'll have to choose one and go with it.

[radarhere]

Just for the record, while there isn't a mechanism for multiple simultaneous mimetypes, there is a way to customise a mimetype based on the image if there is a more specific mimetype that can be used. However, that doesn't look to be the case here - it would appear that there is no distinction between the various Photoshop mimetypes.

[hugovk]

(The s390x failure on Travis is an intermittent Travis problem.)

Thanks!