title: Flask send_file not sending file
body: <p>I'm using Flask with <code>send_file()</code> to have people download a file off the server. </p>
  
  <p>My current code is as follows:</p>
  
  <pre><code>@app.route('/', methods=["GET", "POST"])
  def index():
      if request.method == "POST":
          link = request.form.get('Link')
          with youtube_dl.YoutubeDL(ydl_opts) as ydl:
              info_dict = ydl.extract_info(link, download=False)
              video_url = info_dict.get("url", None)
              video_id = info_dict.get("id", None)
              video_title = info_dict.get('title', None)
              ydl.download([link])
          print("sending file...")
          send_file("dl/"+video_title+".f137.mp4", as_attachment=True)
          print("file sent, deleting...")
          os.remove("dl/"+video_title+".f137.mp4")
          print("done.")
          return render_template("index.html", message="Success!")
      else:
          return render_template("index.html", message=message)
  </code></pre>
  
  <p>The only reason I have <code>.f137.mp4</code> added is because I am using AWS C9 to be my online IDE and I can't install FFMPEG to combine the audio and video on Amazon Linux. However, that is not the issue. The issue is that it is not sending the download request.</p>
  
  <p>Here is the console output:</p>
  
  <pre><code>127.0.0.1 - - [12/Dec/2018 16:17:41] "POST / HTTP/1.1" 200 -
  [youtube] 2AYgi2wsdkE: Downloading webpage
  [youtube] 2AYgi2wsdkE: Downloading video info webpage
  [youtube] 2AYgi2wsdkE: Downloading webpage
  [youtube] 2AYgi2wsdkE: Downloading video info webpage
  WARNING: You have requested multiple formats but ffmpeg or avconv are not installed. The formats won't be merged.
  [download] Destination: dl/Meme Awards v244.f137.mp4
  [download] 100% of 73.82MiB in 00:02
  [download] Destination: dl/Meme Awards v244.f140.m4a
  [download] 100% of 11.63MiB in 00:00
  sending file...
  file sent, deleting...
  done.
  127.0.0.1 - - [12/Dec/2018 16:18:03] "POST / HTTP/1.1" 200 -
  </code></pre>
  
  <p>Any and all help is appreciated. Thanks!</p>