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.

codersdk/chats.go

@@ -5,6 +5,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"io"
+	"mime"
 	"net/http"
 	"net/url"
 	"strings"
@@ -96,6 +97,7 @@ type ChatMessagePart struct {
 	Title       string              `json:"title,omitempty"`
 	MediaType   string              `json:"media_type,omitempty"`
 	Data        []byte              `json:"data,omitempty"`
+	FileID      uuid.NullUUID       `json:"file_id,omitempty" format:"uuid"`
 }
 
 // ChatInputPartType represents an input part type for user chat input.
@@ -103,12 +105,14 @@ type ChatInputPartType string
 
 const (
 	ChatInputPartTypeText ChatInputPartType = "text"
+	ChatInputPartTypeFile ChatInputPartType = "file"
 )
 
 // ChatInputPart is a single user input part for creating a chat.
 type ChatInputPart struct {
-	Type ChatInputPartType `json:"type"`
-	Text string            `json:"text,omitempty"`
+	Type   ChatInputPartType `json:"type"`
+	Text   string            `json:"text,omitempty"`
+	FileID uuid.UUID         `json:"file_id,omitempty" format:"uuid"`
 }
 
 // CreateChatRequest is the request to create a new chat.
@@ -141,6 +145,11 @@ type CreateChatMessageResponse struct {
 	Queued        bool               `json:"queued"`
 }
 
+// UploadChatFileResponse is the response from uploading a chat file.
+type UploadChatFileResponse struct {
+	ID uuid.UUID `json:"id" format:"uuid"`
+}
+
 // ChatWithMessages is a chat along with its messages.
 type ChatWithMessages struct {
 	Chat           Chat                `json:"chat"`
@@ -938,6 +947,42 @@ func (c *Client) GetChatDiffContents(ctx context.Context, chatID uuid.UUID) (Cha
 	return diff, json.NewDecoder(res.Body).Decode(&diff)
 }
 
+// UploadChatFile uploads a file for use in chat messages.
+func (c *Client) UploadChatFile(ctx context.Context, organizationID uuid.UUID, contentType string, filename string, rd io.Reader) (UploadChatFileResponse, error) {
+	res, err := c.Request(ctx, http.MethodPost, fmt.Sprintf("/api/experimental/chats/files?organization=%s", organizationID), rd, func(r *http.Request) {
+		r.Header.Set("Content-Type", contentType)
+		if filename != "" {
+			r.Header.Set("Content-Disposition", mime.FormatMediaType("attachment", map[string]string{"filename": filename}))
+		}
+	})
+	if err != nil {
+		return UploadChatFileResponse{}, err
+	}
+	defer res.Body.Close()
+	if res.StatusCode != http.StatusCreated {
+		return UploadChatFileResponse{}, ReadBodyAsError(res)
+	}
+	var resp UploadChatFileResponse
+	return resp, json.NewDecoder(res.Body).Decode(&resp)
+}
+
+// GetChatFile retrieves a previously uploaded chat file by ID.
+func (c *Client) GetChatFile(ctx context.Context, fileID uuid.UUID) ([]byte, string, error) {
+	res, err := c.Request(ctx, http.MethodGet, fmt.Sprintf("/api/experimental/chats/files/%s", fileID), nil)
+	if err != nil {
+		return nil, "", err
+	}
+	defer res.Body.Close()
+	if res.StatusCode != http.StatusOK {
+		return nil, "", ReadBodyAsError(res)
+	}
+	data, err := io.ReadAll(res.Body)
+	if err != nil {
+		return nil, "", err
+	}
+	return data, res.Header.Get("Content-Type"), nil
+}
+
 func formatChatStreamResponseError(response Response) string {
 	message := strings.TrimSpace(response.Message)
 	detail := strings.TrimSpace(response.Detail)