Are you saving the attachments on to disk or in the db? You can check by going to Settings > System > Files If you are saving them in the db, do you see the binary for the attachment? If not, Filip provides an example on this in an older version of Kentico that may work.
AttachmentInfo newAttachment = new AttachmentInfo
{
AttachmentBinary = imageBinary,
AttachmentDocumentID = 25,
AttachmentExtension = "jpg",
AttachmentSize = imageBinary.Length,
AttachmentName = "newAttachment",
AttachmentMimeType = MimeTypeHelper.GetMimetype("jpg"),
AttachmentTitle = "attachmentTitle",
AttachmentDescription = "",
AttachmentSiteID = CMSContext.CurrentSiteID,
AttachmentGUID = Guid.NewGuid()
};
AddAttachment(
TreeNode node,
string guidColumnName,
Guid attachmentGuid,
Guid groupGuid,
Object file,
TreeProvider tree,
int width,
int height,
int maxSideSize
);
If you are storing the attachments in the file system, Brenden's example should work