The media library can contain files which aren't attached yet. When attaching a file you currently get "Media attachment reattached.", or the opposite "Media attachment detached.". That's redundant and can be inaccurate. It's also not easy to translate in some languages.
This change generalizes the strings to refer to a "media file" instead.
Props SergeyBiryukov, ocean90, netweb.
Fixes#36089.
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