Offline Usage File: Support additional mime-types


## Summary

Customer emailed their offline license usage CSV file to us, and received an automated message back that it could not be processed. 

- [Customer ticket](https://gitlab.zendesk.com/agent/tickets/325524) (internal)  
- [subscription](https://www.zuora.com/apps/Subscription.do?method=view&id=8a12892d8234963001823e7435a3621b)  
- [customer account](https://customers.gitlab.com/admin/customer/437768/edit)


<details><summary>Full details of reported bug</summary>

I see that a [cloud activation](https://customers.gitlab.com/admin/cloud_activation/11787) exists for the subscription, and the [offline license is here](https://customers.gitlab.com/admin/license/1034640).


### Relevant logs and/or screenshots

![screenshot-2022-09-13-15_51](/uploads/7fc191c16e1654c86b9906551ea20caf/screenshot-2022-09-13-15_51.png)

The csv file itself is recorded in the logs here:

https://console.cloud.google.com/logs/query;cursorTimestamp=2022-09-13T09:46:10.532Z;query=resource.type%3D%22gce_instance%22%0A%22enrico.prevosti@2bits.it%22%0Atimestamp%3D%222022-09-13T09:46:10.532Z%22%0AinsertId%3D%221ohz9dwf6pj2i3%22;timeRange=PT8H?project=gitlab-subscriptions-prod

`param[33]` specifically.

I also noticed these log entries from `production.log`

<details>

```
I, [2022-09-13T09:46:10.086326 #969237]  INFO -- : [d94d5079-51c0-4d87-b445-4bb7ddc1f37f] Started POST "/rails/action_mailbox/mailgun/inbound_emails/mime" for 172.70.135.221 at 2022-09-13 09:46:10 +0000
I, [2022-09-13T09:46:10.088610 #969237]  INFO -- : [d94d5079-51c0-4d87-b445-4bb7ddc1f37f] Processing by ActionMailbox::Ingresses::Mailgun::InboundEmailsController#create as */*
I, [2022-09-13T09:46:10.089279 #969237]  INFO -- : [d94d5079-51c0-4d87-b445-4bb7ddc1f37f]   Parameters: {"Accept-Language"=>"it-IT, en-US", ....... [snipped out]

I, [2022-09-13T09:46:10.520825 #969237]  INFO -- : [d94d5079-51c0-4d87-b445-4bb7ddc1f37f]   GCS Storage (385.1ms) Uploaded file to key: [snipped] (checksum: eOh6+mF+m4cU4kqmXYFa1g==)
I, [2022-09-13T09:46:10.522318 #969237]  INFO -- : [d94d5079-51c0-4d87-b445-4bb7ddc1f37f] [ActiveJob] Enqueued ActionMailbox::RoutingJob (Job ID: f2d3a2bb-3291-4802-a2e7-b3321dd92c9b) to Sidekiq(default) with arguments: #<GlobalID:0x00007f8c54b5e798 @uri=#<URI::GID gid://payment-app/ActionMailbox::InboundEmail/30>>
I, [2022-09-13T09:46:10.532055 #969237]  INFO -- : [d94d5079-51c0-4d87-b445-4bb7ddc1f37f] No template found for ActionMailbox::Ingresses::Mailgun::InboundEmailsController#create, rendering head :no_content
I, [2022-09-13T09:46:10.532353 #969237]  INFO -- : [d94d5079-51c0-4d87-b445-4bb7ddc1f37f] Completed 204 No Content in 443ms (ActiveRecord: 19.1ms | Allocations: 25146)

I, [2022-09-13T09:46:10.537933 #969416]  INFO -- : [ActiveJob] [ActionMailbox::RoutingJob] [f2d3a2bb-3291-4802-a2e7-b3321dd92c9b] Performing ActionMailbox::RoutingJob (Job ID: f2d3a2bb-3291-4802-a2e7-b3321dd92c9b) from Sidekiq(default) enqueued at 2022-09-13T09:46:10Z with arguments: #<GlobalID:0x000055749db261c0 @uri=#<URI::GID gid://payment-app/ActionMailbox::InboundEmail/30>>
I, [2022-09-13T09:46:10.630964 #969416]  INFO -- : [ActiveJob] [ActionMailbox::RoutingJob] [f2d3a2bb-3291-4802-a2e7-b3321dd92c9b] ESC[36m  GCS Storage (69.6ms) ESC[0mESC[34mDownloaded file from key: [snipped]ESC[0m
I, [2022-09-13T09:46:10.647535 #969416]  INFO -- : [ActiveJob] [ActionMailbox::RoutingJob] [f2d3a2bb-3291-4802-a2e7-b3321dd92c9b]   Rendered customer_mailer/provisioning_error_notification.html.haml within layouts/mailer (Duration: 4.6ms | Allocations: 3430)
```

</details>

Not sure how relevant it is, but this kinda stands out

>  No template found for ActionMailbox::Ingresses::Mailgun::InboundEmailsController#create, rendering head :no_content

Production's console output:

```
I, [2022-09-13T09:46:10.537933 #969416]  INFO -- : [ActiveJob] [ActionMailbox::RoutingJob] [f2d3a2bb-3291-4802-a2e7-b3321dd92c9b] Performing ActionMailbox::RoutingJob (Job ID: f2d3a2bb-3291-4802-a2e7-b3321dd92c9b) from Sidekiq(default) enqueued at 2022-09-13T09:46:10Z with arguments: #<GlobalID:0x000055749db261c0 @uri=#<URI::GID gid://payment-app/ActionMailbox::InboundEmail/30>>
```

This log line indicates the `ActionMailbox::InboundEmail` object saved from this email has `id = 30`.

Using the steps from [our troubleshooting docs](https://gitlab.com/gitlab-org/customers-gitlab-com/-/blob/main/doc/license/cloud_license.md#troubleshooting), we can see the attached file has `mime-type: 'application/octet-stream'` instead of the [expected](https://gitlab.com/gitlab-org/customers-gitlab-com/-/blob/d2ccaa0f4eab6ff879d4b00a8e5a9ac77db9a9f3/app/mailboxes/offline_provisioning_mailbox.rb#L25) `'text/csv'` so we end up with an *empty* attachment which triggers a [provisioning_error_notification](https://gitlab.com/gitlab-org/customers-gitlab-com/-/blob/main/app/mailboxes/offline_provisioning_mailbox.rb#L14).

<details>
<summary>Output from production</summary>

```ruby
inbound_email = ActionMailbox::InboundEmail.find(30)
=> #<ActionMailbox::InboundEmail id: 30, status: "delivered", message_id: "VI1PR0701MB23836406448561E20F5820D2D5479@VI1PR0701...", ...>

mail = inbound_email.mail
=> #<Mail::Message:91420, Multipart: true, Headers: <Received: from ...>...>

license_usage_file = Array(mail.attachments).find do |attachment|
  attachment.filename == 'license_usage.csv' && attachment.mime_type == 'text/csv'
end
=> nil

# Load all the attachments
attachments = Array(mail.attachments)
attachments.size
=> 2

attachment = attachments.last
=> #<Mail::Part:91460, Multipart: false, Headers: <Content-Type: application/octet-stream; name="license_usage.csv", ...>,

# the csv file is there and contains the usage data but the check for the mime_type doesn't succeed
```
</details>

</details>

## Cause of Bug
In investigating this bug, it was determined that the mime-type `mime-type: 'application/octet-stream'` used by the customer here is not currently supported by our code. This mime-type is somewhat common in email clients.

## Proposal for Resolution
Consider the mime-type `mime-type: 'application/octet-stream'` as part of the [finding condition](https://gitlab.com/gitlab-org/customers-gitlab-com/-/blob/d2ccaa0f4eab6ff879d4b00a8e5a9ac77db9a9f3/app/mailboxes/offline_provisioning_mailbox.rb#L25) for offline usage data reporting.

## Reported examples

1. https://gitlab.zendesk.com/agent/tickets/325524
1. https://gitlab.zendesk.com/agent/tickets/333322
1. https://gitlab.zendesk.com/agent/tickets/335931
1. https://gitlab.zendesk.com/agent/tickets/334735
1. https://gitlab.zendesk.com/agent/tickets/339231
1. https://gitlab.zendesk.com/agent/tickets/340432
1. https://gitlab.zendesk.com/agent/tickets/348986 (mime was `application/vnd.ms-excel`)

<!---
Please add both severity and priority level according to: https://docs.gitlab.com/ee/development/contributing/issue_workflow.html#severity-labels
--->

Story points: 1.0