Hi Itamar- Julianne from Mixpanel Support here!
Diving right in, it isn't possible to upload a JSON file in the UI. However, you could set up a serverless ingest pipeline from a cloud storage bucket into Mixpanel. Once this is set up, you can simply upload files containing events into the designated bucket, and the events will be ingested into Mixpanel, both one-time and on a recurring basis.
If you are using Google Cloud Storage, you can follow this guide to set up the pipeline. If you are using Amazon S3, you can follow this guide. Please note that the JSON files should be formatted according to Mixpanel's event format. Each event must contain an event name, time, distinct_id, and $insert_id.
