fix: recognise Final Draft files by their content

load() chose the parser from the extension alone, so a .txt that is
really FDX was parsed as Fountain. The .fdx extension still wins, and
other names are sniffed for the FinalDraft root element.

CHANGELOG.md

@@ -17,6 +17,8 @@ version is in the `VERSION` file.
 
 - `plotzilla new-script.fountain` opens a new script that will be saved to that file,
   instead of an untitled document that forgot the name it was given.
+- A Final Draft file named `.txt` (or anything but `.fdx`) is recognised by its
+  content and opened as FDX, not parsed as Fountain.
 - The unsaved-changes marker is now updated atomically: an edit landing while a save
   finished could make the close confirmation unreliable.
 - Saving is now durable: the file is synced to disk before it replaces the old one (a

app.go

@@ -118,7 +118,7 @@ func (a *App) load(path string) (*OpenResult, error) {
 		return nil, err
 	}
 	var doc *screenplay.Document
-	if strings.EqualFold(filepath.Ext(path), ".fdx") {
+	if isFDX(path, data) {
 		if doc, err = fdx.Read(bytes.NewReader(data)); err != nil {
 			return nil, err
 		}
@@ -128,6 +128,16 @@ func (a *App) load(path string) (*OpenResult, error) {
 	return &OpenResult{Path: path, Document: doc}, nil
 }
 
+// isFDX reports whether a file is a Final Draft one: the extension says
+// so, or its content does — a ".txt" export of Final Draft is still FDX.
+func isFDX(path string, data []byte) bool {
+	if strings.EqualFold(filepath.Ext(path), ".fdx") {
+		return true
+	}
+	head := strings.TrimPrefix(string(data[:min(len(data), 512)]), "\ufeff")
+	return strings.Contains(head, "<FinalDraft")
+}
+
 // Save writes the document to path, asking for one when empty. It returns
 // the path written, or "" if the user cancelled.
 func (a *App) Save(doc screenplay.Document, path string) (string, error) {