Clarify file upload interface

[marcj303]

It seems that some users have problems with the file uploads. Something about the interface isn't clear. I had two applicants email me their files because they couldn't do it. The description was nebulous, but this has happened to a few people.

[itsrachelfish]

@marcj303 This might have been the same issue that Claire emailed us about while testing. It might not be clear to users that you're supposed to click the "Browse" button to select a file before clicking "Upload"

I added an error message that explains you need to select a file, but we should probably look into changing the interface for uploads to be more clear anyway.

[marcj303]

This helps. I agree, it should not have the upload button until there is a file selected.