How to convert MongoDB archive file to JSON format?

mongoexport

I believe the only way is from the database, using mongoexport:

mongoexport --db yourdb -c yourCollection --out yourCollection.json

warning from mongo

Just take notice of the following (from their website):

WARNING

Avoid using mongoimport and mongoexport for full instance production backups. They do not reliably preserve all rich BSON data types, because JSON can only represent a subset of the types supported by BSON. Use mongodump and mongorestore as described in MongoDB Backup Methods for this kind of functionality.