don't clobber files in `safe/`

with the changes from !14 (merged), safe files have their file extension changed to .pdf. for example: document.docx -> safe/document.pdf

now consider dangerzone processes files with the same basename, but different file extensions: document.docx -> safe/document.pdf and document.doc -> safe/document.pdf. one of these two files will be clobbered. that's fine if both documents are the same contents under a different document format, but it's a problem if they're different documents, something is doing to disappear.

DWP should, on file upload, check if the remote destination already exists. if it does, it should do something similar to firefox, and attempt to upload it as document (1).pdf, document (2).pdf, etc.