import json

with open('urls.har', 'r') as f:
   data = json.load(f)

urls = []
for entry in data['log']['entries']:
   if '/gobackend/mediaserver' in entry['request']['url']:
       params = {q['name']: q['value'] for q in entry['request']['queryString']}
       if 'fp' in params:
           urls.append(params['fp'])

with open('output.txt', 'w') as f:
   for url in urls:
       f.write(url + '\n')