fix: Process non-image inline attachments as regular attachments (#10998)

In Chatwoot, we rely on the Content-ID for inline attachments to replace
the link with the uploaded attachment URL. Our expectation was that only
images would be inline, while other attachments would not. However,
email clients like Apple Mail (sigh) allow users to send inline
attachments that are not images, and these attachments often lack a
Content-ID. This creates significant issues in rendering.
 
I investigated how other email clients handle this scenario. When
viewing the same email (sent from Apple Mail) in Gmail, only one image
appears—and it’s treated as an attachment, not inline. This happens
because both attachments are the same image, and Apple Mail only sends
one copy. See the screenshot below.

| Apple Mail | Gmail | 
| -- | -- | 
| <img width="646" alt="Screenshot 2025-02-27 at 8 20 17 PM"
src="https://github.com/user-attachments/assets/e0d1cd2d-e47c-4081-a53b-7a67106341b3"
/> | <img width="360" alt="Screenshot 2025-02-27 at 8 20 51 PM"
src="https://github.com/user-attachments/assets/b206e56e-8f86-43e9-867b-d895c36aff78"
/> |

A good fix for this would be to check if the Content-ID is missing and
then upload the file as a regular attachment. However, the Mail gem (for
some reason) automatically adds a default Content-ID to inline parts. I
need to dig into the source code to understand why this happens.

For now, I’ve implemented a check to treat non-image attachments as
regular attachments. Inline image attachments are already handled by
appending an image tag at the end if the content-id is not found in the
body. A sample conversation to test this behavior is
[here](https://app.chatwoot.com/app/accounts/1/conversations/46732).

spec/mailboxes/imap/imap_mailbox_spec.rb

@@ -59,6 +59,22 @@ RSpec.describe Imap::ImapMailbox do
       end
     end
 
+    context 'when the email has inline attachments other than images' do
+      let(:inbound_mail) { create_inbound_email_from_fixture('email_with_inline_pdf.eml') }
+
+      it 'creates a conversation and a message with non-image files as regular attachments' do
+        expect do
+          class_instance.process(inbound_mail.mail, channel)
+        end.to change(Conversation, :count).by(1)
+
+        last_message = conversation.messages.last
+        expect(last_message.attachments.count).to be 1
+
+        attachment = last_message.attachments.first
+        expect(attachment.file.blob.filename.to_s).to eq 'dummy.pdf'
+      end
+    end
+
     context 'when the email has 15 or more attachments' do
       let(:inbound_mail) { create_inbound_email_from_fixture('multiple_attachments.eml') }