Fix/mime content type

[preda-bogdan]

Summary

Will get the mime type of the imported image from the Header Content-Type of the image that is downloaded, if the mime_content_type function is available will also use that to ensure the proper type.

This will prevent fatal errors in instances where the mime_content_type function might not be present.

Extended tests to also check for the attachments import.

Will affect the visual aspect of the product

NO

Screenshots

Test instructions

1. Unit tests should pass
2. On a fresh instance where FileInfo PHP extension is not present
3. Check that the import of a feed as an attachment will not trigger an error.

Check before Pull Request is ready:

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• Security and Sanitization requirements have been followed
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)

Closes #909.

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