Add simple HTML template with file upload

Code taken from the following blog posts:
  - https://pythonbasics.org/flask-upload-file
  - https://blog.miguelgrinberg.com/post/handling-file-uploads-with-flask

main.py

@@ -1,13 +1,39 @@
-from flask import Flask
+import os
 
+from flask import Flask, abort, redirect, render_template, request, url_for
+from werkzeug.utils import secure_filename
 
 app = Flask(__name__)
+app.config["MAX_CONTENT_LENGTH"] = 1024 * 1024
+app.config["UPLOAD_EXTENSIONS"] = [".csv"]
+app.config["UPLOAD_PATH"] = "uploads"
 
 
 @app.route("/")
 def index():
+    return render_template("index.html")
+
+
+@app.route("/", methods=["POST"])
+def upload_file():
+    uploaded_file = request.files["file"]
+    filename = secure_filename(uploaded_file.filename)
+
+    if filename != "":
+        file_ext = os.path.splitext(filename)[1]
+        if file_ext not in app.config["UPLOAD_EXTENSIONS"]:
+            abort(400)
+
+        # make sure the upload directory exists
+        try:
+            os.mkdir(app.config["UPLOAD_PATH"])
+        except FileExistsError:
+            pass
+
+        uploaded_file.save(os.path.join(app.config["UPLOAD_PATH"], filename))
+
+    return redirect(url_for("index"))
 
-    return "Congratulations, it's a web app!"
 
 if __name__ == "__main__":
     app.run(host="127.0.0.1", port=8080, debug=True)