minor #16663 [Validator] Improve Image constraint invalid mime type message (mdoutreluingne)

This PR was merged into the 6.1 branch.

Discussion
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[Validator] Improve Image constraint invalid mime type message

Fixes #16662

Changed the default message in case of incorrect mime type for the image constraint.

From 6.1 we will replace `This file is not a valid image.`  with this `The mime type of the file is invalid ({{ type }}). Allowed mime types are {{ types }}.`.

Commits
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4cf3e06 [Validator] Improve Image constraint invalid mime type message

Author: javiereguiluz

reference/constraints/Image.rst

@@ -442,7 +442,7 @@ You can find a list of existing image mime types on the `IANA website`_.
 ``mimeTypesMessage``
 ~~~~~~~~~~~~~~~~~~~~
 
-**type**: ``string`` **default**: ``This file is not a valid image.``
+**type**: ``string`` **default**: ``The mime type of the file is invalid ({{ type }}). Allowed mime types are {{ types }}.``
 
 ``minHeight``
 ~~~~~~~~~~~~~