If the pasted content has strings, we don't upload included files and instead
allow the default behavior to take place. This deals with a quirky behavior of
pastes from MS Word, which in addition to the formatted string content also
includes a thumbnail of it. Images still paste as usual.
(imported from commit 60c4f8dd90ac2e8e38940fb302cc9d1ebeecfdf3)
Don't assume clipboardData.items since it doesn't exist on Safari
Make sure there are no files if using a clipboard drop. Safari includes a blank text/uri-list
data entry
Firefox fix for image pasting
(imported from commit ea0d56fe73ca45cf2e4d437df23a4023bb649445)
Don't show an error if we can't handle the drop contents, since it may
just be empty rather than being a browser unsupported issue
(imported from commit 986495b4a94f4afacf75ffb35ea507d86c369b2f)
