Admin Plugins: Plugin upload without selected file no longer raises exception, but gets caught correctly.
Admin Plugins: Plugin upload without selected file no longer raises exception, but gets caught correctly.

Emtpy argument evaluates to a `unicode` string, while any uploaded content is a `cgi.FieldStorage` object.

Closes #6885.


git-svn-id: eda3d06fcef731589ace1b284159cead3416df9b@6608 af82e41b-90c4-0310-8c96-b1721e28e2e2

walty8/trac,jun66j5/trac-ja,netjunki/trac-Pygit2,netjunki/trac-Pygit2,jun66j5/trac-ja,walty8/trac,jun66j5/trac-ja,walty8/trac,netjunki/trac-Pygit2,jun66j5/trac-ja,walty8/trac