Cached remote meda filenames might collide

[Menrath]

Currently the attachments class is using just the files basename for cached attachments.

This has problems:

• Attachments of the same post might have the same basename and therefore collissions might occur.
• The basename might be guessed and therefore private images might get easily guessed.

I propose a more nuanced way of caching remote media: Use the original URL to generate a hash for the filename.