Use MD5 for attachment ETag

Currently attachments use the revision number of the document they're attached to for their ETag. This can be inefficient if many attachments are added, removed, or modified on the same document. Example: a website's assets might be stored on a document. Whenever one file is changed every file on the website drops out of cache.

If we could use the MD5 of the attachment for the ETag then any caches (including the browser's) will not drop unchanged files, even if the document to which they are attached is modified.