The Android app saves files with the filename format
'signal-YYYY-MM-DD-hhmmss.extension' but the desktop version
saves them with format 'signal-attachment-YYYY-MM-DD-hhmmss.extension'.
The attachment changes the suggested filename in Attachment.ts
to have the prefix 'signal' rather than 'signal-attachment'.
Tests were changed as necessary.
Resolves#4321