CR: Use a less-likely-to-collide thumbnail name for legacy attachments

If you had a legacy file foo.jpg and a file foo-thumbnail.jpg, we'd use
the foo-thumbnail.jpg as the thumbnail for foo.

This isn't a problem with modern attachments as each is in it's own
directory.

// FREEBIE

SignalServiceKit/src/Messages/Attachments/TSAttachmentStream.m

@@ -243,7 +243,7 @@ NS_ASSUME_NONNULL_BEGIN
 
     NSString *filename = filePath.lastPathComponent.stringByDeletingPathExtension;
     NSString *containingDir = filePath.stringByDeletingLastPathComponent;
-    NSString *newFilename = [filename stringByAppendingString:@"-thumbnail"];
+    NSString *newFilename = [filename stringByAppendingString:@"-signal-ios-thumbnail"];
 
     return [[containingDir stringByAppendingPathComponent:newFilename] stringByAppendingPathExtension:@"jpg"];
 }