[Quality/Performance] File Size Calculation for Base64 Encoding and Document Reasoning for 25MB Upload Restriction

[sync-by-unito]

Based from slack thread

1. Make sure user is prevented from uploading more than 25MB
2. Clearly outline the reasons for choosing base64 encoding
3. in the initial implementation
4. and for size calculation

┆Issue is synchronized with this Asana task by Unito
┆Created By: Sam