Problems with metadata input validation

[manuelwedler]

I wanted to verify a contract via metadata but I found two problems in the process:

1. When pasting a file, you have to name the metadata with a .sol ending.
   IMO, the file name field should be completely removed for the metadata file.

2. I have to provide additional sources even though all sources are already in the metadata under content properties.
   It should not be a requirement to provide additional sources.