resolveAttachmentFiles
Resolves deferred attachments by copying their source content into local cache files.
Attachments that already have a non-null Attachment.upload are returned unchanged. For others, the original content URI is read from EXTRA_SOURCE_URI and copied to a local cache file. Attachments whose source URI cannot be resolved (e.g. the content URI is no longer accessible or the cache write fails) are dropped from the result.
Return
Attachments with Attachment.upload populated for every entry that had a source URI.
Parameters
attachments
The attachments to resolve.