Binary files in problem editing

[cayhorstmann]

When editing a problem that has a binary file (such as hsql.db in Gerald Cohen's https://codecheck.io/files/22121513531ga6ejmwq41uwlkw8tudtitd2), then the binary file is shown in a textbox and probably corrupted when resubmitting. Allow for uploading them, or provide a message that they can't be edited.