Apply the upload limit only where something is uploaded

FEATURES has always said attach-from-Files works "however large", because a
blob the account already holds is attached by reference and nothing is
sent. The code checked every file against maxSizeUpload regardless, so the
two disagreed and the code was the one that was wrong.

maxSizeUpload is what the server will accept for a single upload (RFC
8620). It bears on a file that is about to be uploaded and on nothing
else. Applying it to a by-reference attachment refused a 60 MB message the
server was already storing, on the grounds that it could not have been
uploaded -- which it was not being. Forwarding a large message as an
attachment hit exactly that.

A file from somebody else's account is fetched and re-uploaded into this
one, because a message can only carry blobs from the account sending it.
That upload is real and the limit is real for it, so it still applies
there.

FEATURES.md

@@ -459,6 +459,13 @@ minimisable and maximisable; full-screen on mobile.
   upload at all**, however large. A file from someone else's shared folder is
   copied to your account first, because a message can only carry blobs from the
   account sending it; the picker says so before it does.
+
+  The upload limit applies to that copy and to nothing else. `maxSizeUpload` is
+  what the server will accept for a single *upload*, so it bears only on a file
+  that is about to be uploaded — a blob this account already holds is attached
+  by reference and never sent. Checking it in both cases refused a 60 MB message
+  the server was already storing, on the grounds that it could not have been
+  uploaded, which it was not being.
 - **Attachment reminder** when the text mentions an attachment and none is there.
 - **Spell check** toggle.
 - **Drafts** save as you type and on close, with the save state shown.