Increase test coverage

### How do you use Sentry?

Sentry Saas (sentry.io)

### Version

all

### Steps to Reproduce

Those files have very low test coverage and we should revise the tests and improve the coverage:
- `attachments.py`
- `utils.py`
- ~~`worker.py (background worker to send events)`~~
    - Split off into #4402

### Expected Result

Test coverage as high as possible

### Actual Result

Low test coverage