Function getMediaAttachmentFromFileEvent

Extracts a MIP-04 v2 attachment from a NIP-94 kind 1063 file-metadata event.

Kind 1063 events use flat tags (url, m, x, filename, n, v, …) rather than the space-separated imeta format. Standard NIP-94 fields are parsed by applesauce's getFileMetadata; the MIP-04-specific fields (filename, n, v) are read directly from the flat tag list.

Returns null if:

• The v tag is absent or does not match MIP04_VERSION
• The n (nonce) tag is absent or is not exactly 24 characters (hex-encoded 12-byte nonce)
• The filename tag is absent or empty
• The x (sha256) tag is absent or is not exactly 64 characters (hex-encoded 32-byte hash)
• The m (MIME type) tag is absent or is not a valid type/subtype string