* MM-63649 - fix preview embeded images
* enhance utils function and simplify validations in file preview modal component
* add new test for proxied images without extension
* simplify logic; apply DRY unifying it via helper function
* if extension is available in file info, use it first
* add extra validation for the extension lenght if fileInfo.extension is present
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Co-authored-by: Mattermost Build <build@mattermost.com>
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