attachments created by PostAttachment method cannot be viewed by customers

## What would you like to be added?
- IssueService method PostAttachment creates attachment which cannot be viewed by customers. It would be nice to be able to set visibility for customer also (maybe even as default). (This exists for comments based on role. Default is public for customer).
- Attachment Type has no visibility field
```
type Attachment struct {
Self string `json:"self,omitempty" structs:"self,omitempty"`
ID string `json:"id,omitempty" structs:"id,omitempty"`
Filename string `json:"filename,omitempty" structs:"filename,omitempty"`
Author *User `json:"author,omitempty" structs:"author,omitempty"`
Created string `json:"created,omitempty" structs:"created,omitempty"`
Size int `json:"size,omitempty" structs:"size,omitempty"`
MimeType string `json:"mimeType,omitempty" structs:"mimeType,omitempty"`
Content string `json:"content,omitempty" structs:"content,omitempty"`
Thumbnail string `json:"thumbnail,omitempty" structs:"thumbnail,omitempty"`
}
```

## Why is this needed?
Most tickets are created through the Jira Portal. It stands to reason, that any work processed for a ticket that produces an attachment result should be visible to the reporter.

## Anything else we need to know?
N/A