Load a profile from URL doesn't work for perf script profiles

Profiles generated using `perf script` can be opened from the file selection dialog, but loading the same file by URL fails with:

```
The profile could not be downloaded and decoded. This does not look like a supported file type.
JSON parsing error: SyntaxError: JSON.parse: unexpected character at line 1 column 1 of the JSON data
```

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-493)