NGINX: Reverse proxy gives 405 Not Allowed when doing POST

Issue

Flask app with UI failed to access when setup with nginx show error: 405 Not Allowed. Here is the config:

        location /rmbg {
          proxy_pass http://127.0.0.1:5100/;
          proxy_set_header X-Real-IP $remote_addr;
          proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
          proxy_set_header X-NginX-Proxy true;
          proxy_set_header Host $host;
          proxy_redirect off;
        }

Cause

Html form action for post is still located at original location /

      <form
        id="uploadForm"
        action="/"
        method="post"
        enctype="multipart/form-data"
        >
        <input id="fileInput" type="file" name="file" />
        <input type="submit" value="rm" />
      </form>

Resolution

Change location match with nginx:

      <form
        id="uploadForm"
        action="/rmbg"
        method="post"
        enctype="multipart/form-data"
        >
        <input id="fileInput" type="file" name="file" />
        <input type="submit" value="rm" />
      </form>

References:

• stackoverflow.com/questions/76833614/nginx-..
• stackoverflow.com/a/12179750/1377004