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Props mukesh27, peterwilsoncc, azaozz, apermo, flixos90, ironprogrammer.

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Test this pull request with WordPress Playground.

Just tested this again (manually) and all seems to work properly.

• The name of the originally uploaded HEIC image is preserved after converting it, only the extension is different.
• If a file with the target name already exists, the name of the uploaded image is properly incremented. I.e. if image.jpg already exists in the uploads directory, and the user uploads image.heic, the latter is saved as image-1.heic, and all resulting sub-sizes and/or converted images have the -1 suffix.

Merged r59379 / e1f052e

Thanks all for helping get this over the line.