fix: some attachments have invalid filenames, and duplicated attachment names overwrite eachother

[caesay]

Closes #2

Per title. There were two bugs here:

• When attachments have invalid filenames they can not be written to disk and this stops the whole process
  • When there is a \ or / char in the filename, it causes Error: ENOENT: no such file or directory #2 since this is treated as a "folder" but we have not created the folder.
• When attachments from different emails have the same filename, they get overwritten in the output folder.
  • Now we check if the filename already exists, and if it does we write it to a new file with _1, _2, etc appended to the filename.