Make paste as .txt attachment threshold configurable

TypingMind does a great job detecting extremely long pasted text and turning it into a .txt attachment automatically. Unfortunately, the current trigger point is set far too high—about 30000 characters (≈ 800 lines of code). Most real-world snippets (logs, stack traces, medium-sized source files, etc.) fall well below that limit yet still clutter the chat window and slow down the UI when pasted directly.

Please consider to lower the default threshold or make it available for user to custom threshold