Preserve file extensions for generic attachments #6232

[RomelaGG]

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Description

Fixes #6232

This fixes generic attachment exports losing their original filename extension when saved or shared.

Previously, AttachmentStream.makeDecryptedCopy(filename:) preferred the MIME-derived extension before the sender-provided filename extension. For generic file attachments this can be lossy, for example:

• .srt attachments may have text/plain MIME type and save as .txt
• .ipa attachments may have application/octet-stream MIME type and save as .bin

For attachments whose validated content type is .file, this change preserves the sender's filename extension when available. Media attachments still prefer MIME-derived extensions, preserving the existing behavior for images, videos, audio, and animated images.

I also replaced URL(string:) extension extraction with NSString.pathExtension, which is safer for normal filenames that contain spaces or non-URL-safe characters.

Testing:

• Confirmed git diff --check passes.
• Manually tested saving .srt and .ipa attachments to Files; both preserve their original file extensions.