[Vanilla Bug] Make side-specific mix-files take priority

[Bittah]

First Check

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Description

Currently the files in the side-specific mix-files don't take priority, which means that a file that for example exists in both sidec##.mix and ecache##.mix will only be read from ecache##.mix and are only downsides to this. If the side-specific mix-files would take priority it'd be possible to for example have one file in ecache##.mix that's shared by 3 factions and to then place another file with the same name in for sidec04.mix so that only the 4th faction will use the different file.

Proposed Code (optional)

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Additional Context

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