Fix MMS returns Bad Request

Existing code tried to replace attachments with a text string,
irrespective of whether the mime part was an attachment or not. This
caused MAP to return Bad Request when mime part was text content type.

Also removed a redundant newline when an attachment is converted to a
text string.

Fixes: 178589673
Tag: #stability
Test: atest BluetoothInstrumentationTests
Test: Verify phone no longer returns a "sent Bad Request" obex
error in HCI logs when attempting a Get "x-bt/message" on a
plain text MMS.

Change-Id: I2b87746e45f8f02dbfe37eee0f5620d92ecad106