The bug has been opened on https://bugs.launchpad.net/ubuntu/+source/nautilus/+bug/126616

"Binary package hint: evolution

Using:
    Ubuntu Feisty
    Evolution 2.10.1
    Nautilus 2.18.1 File Browser

It appears that folders in Evolution are valid destinations to drop the files, but they never actually get there.
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There exists a PDF file in /home/david that I would like to drag and drop into an Evolution folder using Ubuntu Feisty's default Nautilas file browser. The cursor changes to a plus sign to indicate that the Evolution folder is a valid location to drop the file. However, the file does not get copied into the folder. Using the same procedure to copy files between two Evolution folders works perfectly.
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Storing documents in Evolution is a habit acquired from my days as a Windows programmer where we used Exchange Server as the document management system. It provided more access control options, and since e-mails between programmers often referred to documents, it was convenient to have both in the same data store.
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I'd like to second this bug: dragging a file from Nautilus into the Evolution window is possible. Drag any file onto either a folder in the folder list (but not a search folder, junk or deleted items), and you see that it is accepted (a plus-sign appears on the dragged file and the folder is (kind of) selected (dotted rectangle around it).

This action only does something (ok, I didn't look in the source, so there may be exceptions) when the dragged file is an mbox. In that case, the email messages inside are copied into the folder. (This also works the other way around: dragging an email message from evolution into Nautilus creates an mbox for that message.)

When the dragged file is not an mbox, nothing happens. This is confusing: either it should not be possible to drag a not-accepted file into Evolution, or a message should be displayed that it is not a supported file type (or something like that)."