[browser] "No Downloads available" message is confusing in picker mode

(originally reported in bug #1528651)

When picking a file, if selecting the browser as a content source, the downloads view is shown with checkboxes in front of downloads to select which files to upload. If the input widget specifies an "accept" attribute to filter in only a certain set of file types, and if the downloads folder doesn’t have any matching file, the user is greeted with this message:

    "No Downloads available"

This may be confusing, as there may be actual downloads in the folder, even though none matches the accept attribute.

As a concrete example, if the download views has e.g. PDF documents but no images and the user visits https://www.wufoo.com/html5/attributes/07-accept.html (which accepts only images), the view will be empty, but the message implies that this is because there are no downloads at all.

I would suggest a different message in that case, something like:

    "No Downloads matching the expected file type(s)"

or

    "No Downloads matching the expected content type(s)"