User can't upload content pack due to json format error

## Expected Behavior
JSON format of the downloaded content pack is correct and the user able to upload it

## Current Behavior
When the user downloads the content pack from the list then it's impossible to upload it

https://user-images.githubusercontent.com/10741557/159505966-b61692ce-8027-4d75-9889-6c7bd8e3596f.mp4

## Possible Solution
Looks like there is a problem with JSON format when the user downloads the content pack from the list of packs. This problem does not exist when a user downloads a content pack while creating.

## Steps to Reproduce (for bugs)
1. go to `system/contentpacks`
2. choose any of the existing content pack
3. click `More Action `and then download the pack
4. Click `Upload` button and choose the downloaded content pack
5. You will see an error that there is a problem with JSON format and it's impossible to upload pack

## Your Environment

* Graylog Version: Graylog 4.3.0-beta.2+a61a34c
* Operating System: Mac OS Monterey 12.2.1 (21D62)
* Browser version: Chrome Version 99.0.4844.74 (Official Build) (arm64)