admin: improve files and stats display

[carlinmack]

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Close #2898

Description

Please describe briefly your pull request.

Changes:

1. Reorder files from Size | Number to Number: Size
2. Change from MiB -> MB (etc). I think MB is more standard? But we should confirm first
3. Remove extra separator on the stats column
4. See below

4. Add pop over to explain
   

Checklist

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