Fix image & video cache creating new entries when selecting data without explicit media type

[Wumpf]

What

• Fixes Selecting a video takes up the memory of that video _again_ #7584
  • but also affects images, just not as noticeable there until you kept a blob selected for a long time

That lengthy title is best explained in steps:

• load a large video (or image!) by drag & droping it in
• select it
• observe how memory goes up again!

This happened because image & video both use the media type as part of their cache key entries, but we tend to sometimes resolve it ahead of time and sometimes not, so we get new cache entries.
We do want the media type in the cache entries so we can memorize which (user selected / override affected) media types won't work, but we have to make sure we don't waste memory because of when we resolve the media type.

Before:

before.mp4

After:

after.mp4

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[teh-cmc]

I'm not sure why some things got renamed to MIME (in this PR, and previous ones)?

Not only is it inconsistent with the rest of the codebase which now sometimes calls things MIME and other times MediaType, but also these are certainly media types.

[Wumpf]

I'm not sure why some things got renamed to MIME

Tried to make a few of the errors more consistent 🤷