Whisk oil, vinegar, coriander, salt, pepper, and cardamom in a large bowl. Add cantaloupe, cucumber, and chiles and toss to coat in dressing. Let sit, uncovered, 15 minutes.
To serve, add pumpkin seeds, cilantro, and mint to salad and toss gently to combine. Top with sumac.
Recipe Notes
Sumac, a tart, citrusy spice generally sold in ground form, can be found at Middle Eastern markets, specialty foods stores, and online.