Follow-up to #6174

When a user downloads a PDF - and this PDF equals to an already attached PDF, it should not be attached again.

Possible steps:

1. default.pdf exists
2. Download as default (1).pdf.
3. Check if file content of default.pdf equals file content of default (1).pdf (Use https://guava.dev/releases/21.0/api/docs/com/google/common/io/Files.html#equal-java.io.File-java.io.File-)
4. If it is equal, just delete default (1).pdf. Show notification that already linked. Finish.
5. If not equal, add to linked files.