Use the right field to check if a document exists with a given file name

To check if a document exists with a given file name, FileManagerUtils#getExistingDocByFileName uses "file:filename", whereas it should use the "file:content/name" sub-property, which we can always rely on.

Indeed, currently when uploading an image with the same file name as an existing one in a given folder using the "Import a file" button, a new document is created instead of a new version of the existing document. This happens because the check is done on "file:filename" which is not set in the case of an image (see image_widget_template.xhtml).
Yet it works with a regular file because this field is set, see extended_file_widget.xhtml.