Slack import fails in Mattermost 1.3?

Hi @Sajuka, appreciate your report, could we ask your help:

1. Setting up a Mattermost instance with just default settings and attempt to run your Slack import?

2. If issues persist, is there a way you could remove any information you feel confidential from your Slack import file and share it so that the issue you're experiencing can be reproduced?

There could be an issue with either Slack changing the format in which is stores your data or it could be an issue with the Slack import procedure, or certain configurations.

1 and 2 would help narrow down the possibilities.