"Failed_To_upload_Import_File" for Slack Import

Description:

When attempting to import a .zip file with Slack history, an error message always pops up in the upper right hand corner saying ""Failed_To_upload_Import_File".
This error does not produce a log entry.
Regular file uploads work well.

Steps to reproduce:

Go to 'Administration' > 'Import' > 'Slack' > either 'Choose File' or use a 'File URL'
Both options result in the same error.

Expected behavior:

Slack history should be uploaded to the server.

Actual behavior:

An error message always pops up in the upper right hand corner saying ""Failed_To_upload_Import_File".

Screenshot

Server Setup Information:

• Version of Rocket.Chat Server: Latest Docker image
• Operating System: Debian Stretch
• Deployment Method: Docker
• Number of Running Instances: 1
• DB Replicaset Oplog:
• NodeJS Version: node:8.11-slim (from Rocket.Chat Docker image)
• MongoDB Version: MongoDB shell version v4.0.5

Additional context

I am evaluating Rocket.Chat to use at our company instead of Slack and running into and issue when trying to import Slack history.
I have set up a self-hosted instance of Rocket.Chat on our server which runs Debian Stretch. Both Rocket.Chat and the MongoDB are running in Docker containers and everything is working smoothly. Registration, chat, file uploads (GridFS), encryption, backups, etc

POST method fails with 413 status Payload Too Large when trying to import a slack .zip file.
However, the file is less than 1MB and I have successfully tested uploads on 50+MB files.

Relevant logs:

Server does not produce a log entry when the error occurs.
Browser produces 413 status Payload Too Large.

Start at Administration > Import > Slack and reproduce both the Choose File and File URL flows with the reported Slack .zip. Inspect the POST request and its 413 response, then trace the import upload entry point to identify why a file under 1 MB is rejected. Done means the Slack archive uploads and imports successfully without the Failed_To_upload_Import_File message.