files.json shouldn't use $autoIncrement

[3vorp]

files.json currently uses the $autoIncrement feature of Firestorm, which it really shouldn't for a few reasons:

• The key name is insignificant
• Can lead to key collisions if two requests are made too close to each other
• Inconsistent with all other collections
• Can be confused with add-on IDs (not as much of an issue but still)

Since $autoKey is already enabled, API changes would probably be fairly minimal (since the key was never significant to begin with)—the main thing would be switching the JSON and the config and making sure nothing breaks.