Initial work.

.env.example

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+export SLACK_TEAM=
+export SLACK_COOKIE=
+export SLACK_CRUMB=

upload.py

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+# Upload files named on ARGV as Slack emoji.
+
+import os
+import sys
+import requests
+
+team_name = os.getenv('SLACK_TEAM')
+cookie = os.getenv('SLACK_COOKIE')
+crumb = os.getenv('SLACK_CRUMB')
+
+url = "https://{}.slack.com/customize/emoji".format(team_name)
+
+for filename in sys.argv[1:]:
+    print("Processing {}.".format(filename))
+
+    emoji_name = os.path.basename(filename)
+
+    headers = {
+        'Cookie': cookie,
+    }
+    data = {
+        'add': 1,
+        'crumb': crumb,
+        'name': emoji_name,
+        'mode': 'data',
+    }
+    files = {'img': open(filename, 'rb')}
+    r = requests.post(url, headers=headers, data=data, files=files, allow_redirects=False)
+    r.raise_for_status()