Preheat oven to 250°. Place oxtails in a medium bowl and season generously with salt. Sprinkle with curry powder and toss to coat completely. Heat oil in a medium Dutch oven or other heavy pot over medium-high. Cook oxtails, turning as needed, until deeply browned all over, 10–12 minutes. Add broth, scraping up any browned bits on bottom of pot. Add scallions, garlic, chiles (if using), thyme, ginger, soy sauce, brown sugar, and allspice. Bring to a simmer, partially cover pot, and transfer to oven. Braise oxtails, turning once or twice and adding a splash of water if pot is looking dry, until fork-tender and meat is beginning to fall apart, about 7 hours.