Add support for uploading SVGs

[LukeTowers]

Need to integrate with https://github.com/darylldoyle/svg-sanitizer to automatically sanitize SVGs on upload and then re-add them to the list of allowed defaultExtensions, assetExtensions, and imageExtensions.

Original october#5261

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[github-actions]

This issue will be closed and archived in 3 days, as there has been no activity in the last 60 days.
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[qroft]

Hi Luke,
i am too noobish to know if what i found out fits into your request for integrating the sanitizer or if this is a bug
that i have encountered.
I have allowed the SVG file type inside the cms.php configuration.
When i try to upload a SVG file through the Media manager A) i do not get an error B) the file is not listed and uploaded.

So as a workaround i thought: let's upload my 6 SVG files manually via FTP.
What happens is A) the files are uploaded B) the Media manager shows me "6 item(s)" to the right of the folder but when i C) enter the folder, no images are shown.

Now when i call the full path of the uploaded SVG, it is shown.
In short: the SVG is uploaded and can be used, but the Media manager does not show it.