fix: align chat attachment picker with allowed file types (#24917)

The agent chat composer only advertised image uploads to the OS file
picker and filtered drag-and-drop and paste events to `image/*`, even
though the backend accepts text, CSV, JSON, PDF, and a narrower set of
image types.

Move the allowed chat attachment media types into `codersdk` so the
frontend picker and backend enforcement share one source of truth. Use
the generated TypeScript list to drive the file input `accept` attribute
and the drag-and-drop and paste filters, while adding common text
extensions so platforms without MIME registrations still surface those
files in the picker.

coderd/x/chatfiles/mime.go

@@ -13,6 +13,8 @@ import (
 
 	"github.com/gabriel-vasile/mimetype"
 	"golang.org/x/xerrors"
+
+	"github.com/coder/coder/v2/codersdk"
 )
 
 const MaxStoredFileNameBytes = 255
@@ -28,17 +30,17 @@ var (
 
 	utf8BOM = []byte{0xEF, 0xBB, 0xBF}
 
-	allowedStoredMediaTypes = map[string]struct{}{
-		"image/png":        {},
-		"image/jpeg":       {},
-		"image/gif":        {},
-		"image/webp":       {},
-		"text/plain":       {},
-		"text/markdown":    {},
-		"text/csv":         {},
-		"application/json": {},
-		"application/pdf":  {},
-	}
+	// allowedStoredMediaTypes is derived from codersdk.AllChatAttachmentMediaTypes
+	// so the frontend file picker and the server enforcement share a single
+	// source of truth. Do not edit this map directly; add new entries to the
+	// codersdk const block instead.
+	allowedStoredMediaTypes = func() map[string]struct{} {
+		m := make(map[string]struct{}, len(codersdk.AllChatAttachmentMediaTypes))
+		for _, t := range codersdk.AllChatAttachmentMediaTypes {
+			m[string(t)] = struct{}{}
+		}
+		return m
+	}()
 
 	recordingArtifactMediaTypes = map[string]struct{}{
 		"video/mp4":  {},
@@ -139,14 +141,16 @@ func PrepareRecordingArtifact(name, expectedMediaType string, data []byte) (stor
 
 // IsCompatibleUploadMediaType reports whether an upload request that declared
 // declaredMediaType may be stored as storedMediaType after byte
-// classification. Exact matches are always compatible; the compatibility
-// table only covers explicit refinements like text/plain uploads that safely
-// store as richer text subtypes.
+// classification. Exact matches are always compatible. Clients that declare
+// application/octet-stream are treated as "unknown", so the classified bytes
+// decide the stored type. The compatibility table also covers explicit
+// refinements like text/plain uploads that safely store as richer text
+// subtypes.
 func IsCompatibleUploadMediaType(declaredMediaType, storedMediaType string) bool {
 	declaredMediaType = BaseMediaType(declaredMediaType)
 	storedMediaType = BaseMediaType(storedMediaType)
 
-	if declaredMediaType == storedMediaType {
+	if declaredMediaType == storedMediaType || declaredMediaType == "application/octet-stream" {
 		return true
 	}
 	if declaredMediaType != "text/plain" {

coderd/x/chatfiles/mime_test.go

@@ -270,6 +270,18 @@ func TestIsCompatibleUploadMediaType(t *testing.T) {
 			stored:   "text/plain",
 			want:     true,
 		},
+		{
+			name:     "OctetStreamMatchesPNG",
+			declared: "application/octet-stream",
+			stored:   "image/png",
+			want:     true,
+		},
+		{
+			name:     "OctetStreamMatchesJSON",
+			declared: "application/octet-stream",
+			stored:   "application/json",
+			want:     true,
+		},
 		{
 			name:     "TextPlainRefinesToMarkdown",
 			declared: "text/plain",