Different files in folder content [#3574784]

Problem/Motivation

The Importer, in importContent() method, give as default that a file in the content folder can be json or yml.
if other type of file is in the folder, give error in Yaml::decode($contents)

Proposed resolution

add an additional check on the extension

Issue fork default_content-3574784

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2.0.x changes, plain diff MR !66
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3574784-different-files-in compare
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