Add MIMEType application/x-prop (#545)

Example:
`GET https://vrchat.com/api/1/file/file_c991c050-8b43-4046-8182-24d068524ac5 -> {"extension":".vrcp",...,"mimeType":"application/x-prop","name":"Prop - DroneCube - Asset bundle - 2022․3․22f1_4_standalonewindows_Release",...}`

Author: VinyarionHyarmendacil

openapi/components/schemas/MIMEType.yaml

@@ -4,6 +4,7 @@ enum:
   - application/gzip
   - application/octet-stream
   - application/x-avatar
+  - application/x-prop
   - application/x-rsync-delta
   - application/x-rsync-signature
   - application/x-world