Fix SLACK_FILE_UPLOAD with correct SLACK_TOKEN usage for files.upload

[DEVANSH-GAJJAR]

Problem

The SLACK_FILE_UPLOAD logic in the current code uses SLACK_BOT_TOKEN, but this variable is not always set or consistent with the rest of the workflow, which primarily uses SLACK_TOKEN.

Solution

• Replaced SLACK_BOT_TOKEN with SLACK_TOKEN for file upload authorization.
• Ensured consistency with the main Slack messaging logic which already uses SLACK_TOKEN.
• Added a check to show an error message if SLACK_TOKEN is not provided.

Related Issue

Closes #178

[DEVANSH-GAJJAR]

Hii there , i have submitted a PR to resolve the issue #178 . Please let me know if there is any changes to it needed . Thanks