feat: add file/image attachment support to chat input (#22604)

This change adds support for image attachments to chat via add button
and clipboard paste. Files are stored in a new `chat_files` table and
referenced by ID in message content. File data is resolved from storage
at LLM dispatch time, keeping the message content column small.

Upload validates MIME types via content type or content sniffing against
an allowlist (png, jpeg, gif, webp). The retrieval endpoint serves files
with immutable caching headers. On the frontend, uploads start eagerly
on attach with a background fetch to pre-warm the browser HTTP cache so
the timeline renders instantly after send.

coderd/chatd/chatd.go

@@ -165,6 +165,9 @@ type CreateOptions struct {
 	ModelConfigID      uuid.UUID
 	SystemPrompt       string
 	InitialUserContent []fantasy.Content
+	// ContentFileIDs maps content block indices to their chat_files IDs
+	// so the file_id can be preserved in the stored message JSON.
+	ContentFileIDs map[int]uuid.UUID
 }
 
 // SendMessageBusyBehavior controls what happens when a chat is already active.
@@ -180,10 +183,11 @@ const (
 
 // SendMessageOptions controls user message insertion with busy-state behavior.
 type SendMessageOptions struct {
-	ChatID        uuid.UUID
-	Content       []fantasy.Content
-	ModelConfigID *uuid.UUID
-	BusyBehavior  SendMessageBusyBehavior
+	ChatID         uuid.UUID
+	Content        []fantasy.Content
+	ContentFileIDs map[int]uuid.UUID
+	ModelConfigID  *uuid.UUID
+	BusyBehavior   SendMessageBusyBehavior
 }
 
 // SendMessageResult contains the outcome of user message processing.
@@ -199,6 +203,7 @@ type EditMessageOptions struct {
 	ChatID          uuid.UUID
 	EditedMessageID int64
 	Content         []fantasy.Content
+	ContentFileIDs  map[int]uuid.UUID
 }
 
 // EditMessageResult contains the updated user message and chat status.
@@ -278,7 +283,7 @@ func (p *Server) CreateChat(ctx context.Context, opts CreateOptions) (database.C
 			}
 		}
 
-		userContent, err := chatprompt.MarshalContent(opts.InitialUserContent)
+		userContent, err := chatprompt.MarshalContent(opts.InitialUserContent, opts.ContentFileIDs)
 		if err != nil {
 			return xerrors.Errorf("marshal initial user content: %w", err)
 		}
@@ -345,7 +350,7 @@ func (p *Server) SendMessage(
 		return SendMessageResult{}, xerrors.Errorf("invalid busy behavior %q", opts.BusyBehavior)
 	}
 
-	content, err := chatprompt.MarshalContent(opts.Content)
+	content, err := chatprompt.MarshalContent(opts.Content, opts.ContentFileIDs)
 	if err != nil {
 		return SendMessageResult{}, xerrors.Errorf("marshal message content: %w", err)
 	}
@@ -448,7 +453,7 @@ func (p *Server) EditMessage(
 		return EditMessageResult{}, xerrors.New("content is required")
 	}
 
-	content, err := chatprompt.MarshalContent(opts.Content)
+	content, err := chatprompt.MarshalContent(opts.Content, opts.ContentFileIDs)
 	if err != nil {
 		return EditMessageResult{}, xerrors.Errorf("marshal message content: %w", err)
 	}
@@ -1607,6 +1612,25 @@ func (p *Server) subscribeChatControl(
 	return controlCancel
 }
 
+// chatFileResolver returns a FileResolver that fetches chat file
+// content from the database by ID.
+func (p *Server) chatFileResolver() chatprompt.FileResolver {
+	return func(ctx context.Context, ids []uuid.UUID) (map[uuid.UUID]chatprompt.FileData, error) {
+		files, err := p.db.GetChatFilesByIDs(ctx, ids)
+		if err != nil {
+			return nil, err
+		}
+		result := make(map[uuid.UUID]chatprompt.FileData, len(files))
+		for _, f := range files {
+			result[f.ID] = chatprompt.FileData{
+				Data:      f.Data,
+				MediaType: f.Mimetype,
+			}
+		}
+		return result, nil
+	}
+}
+
 func (p *Server) processChat(ctx context.Context, chat database.Chat) {
 	logger := p.logger.With(slog.F("chat_id", chat.ID))
 	logger.Info(ctx, "processing chat request")
@@ -1922,7 +1946,7 @@ func (p *Server) runChat(
 		p.maybeGenerateChatTitle(context.WithoutCancel(ctx), chat, messages, model, providerKeys, logger)
 	}()
 
-	prompt, err := chatprompt.ConvertMessages(messages)
+	prompt, err := chatprompt.ConvertMessagesWithFiles(ctx, messages, p.chatFileResolver())
 	if err != nil {
 		return xerrors.Errorf("build chat prompt: %w", err)
 	}
@@ -2064,7 +2088,7 @@ func (p *Server) runChat(
 		}
 
 		if len(assistantBlocks) > 0 {
-			assistantContent, err := chatprompt.MarshalContent(assistantBlocks)
+			assistantContent, err := chatprompt.MarshalContent(assistantBlocks, nil)
 			if err != nil {
 				return err
 			}
@@ -2270,7 +2294,7 @@ func (p *Server) runChat(
 			if err != nil {
 				return nil, xerrors.Errorf("reload chat messages: %w", err)
 			}
-			reloadedPrompt, err := chatprompt.ConvertMessages(reloadedMsgs)
+			reloadedPrompt, err := chatprompt.ConvertMessagesWithFiles(reloadCtx, reloadedMsgs, p.chatFileResolver())
 			if err != nil {
 				return nil, xerrors.Errorf("convert reloaded messages: %w", err)
 			}
@@ -2363,7 +2387,7 @@ func (p *Server) persistChatContextSummary(
 			ToolName:   "chat_summarized",
 			Input:      string(args),
 		},
-	})
+	}, nil)
 	if err != nil {
 		return xerrors.Errorf("encode summary tool call: %w", err)
 	}