Use file_exists to check for an uploaded file.

It turns out that stream_resolve_include_path doesn't work for a default
Windows install where upload_tmp_dir is set to C:\Windows\Temp. I'm
unclear if this is intended PHP behaviour or not, but the upload file
validator should not be checking for a file of the same name on the
include path regardless.
Use is_file() rather than file_exists().
Merge pull request #6664 from akrabat/hotfix/uploadfile

Use file_exists to check for an uploaded file.
Merge branch 'hotfix/6664'

Close #6664