Fix detect.py URL inference (ultralytics#5525)

* Fix detect.py URL inference

Allows detect.py to run inference on remote URL sources, i.e.:

```python
!python detect.py --weights yolov5s.pt --source https://ultralytics.com/assets/zidane.jpg  # image URL
!python detect.py --weights yolov5s.pt --source https://ultralytics.com/assets/decelera_landscape.mov  # video URL
```

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detect.py

@@ -24,10 +24,10 @@
 ROOT = Path(os.path.relpath(ROOT, Path.cwd()))  # relative
 
 from models.experimental import attempt_load
-from utils.datasets import LoadImages, LoadStreams
-from utils.general import (LOGGER, apply_classifier, check_img_size, check_imshow, check_requirements, check_suffix,
-                           colorstr, increment_path, non_max_suppression, print_args, save_one_box, scale_coords,
-                           strip_optimizer, xyxy2xywh)
+from utils.datasets import IMG_FORMATS, VID_FORMATS, LoadImages, LoadStreams
+from utils.general import (LOGGER, apply_classifier, check_file, check_img_size, check_imshow, check_requirements,
+                           check_suffix, colorstr, increment_path, non_max_suppression, print_args, save_one_box,
+                           scale_coords, strip_optimizer, xyxy2xywh)
 from utils.plots import Annotator, colors
 from utils.torch_utils import load_classifier, select_device, time_sync
 
@@ -61,8 +61,11 @@ def run(weights=ROOT / 'yolov5s.pt',  # model.pt path(s)
         ):
     source = str(source)
     save_img = not nosave and not source.endswith('.txt')  # save inference images
-    webcam = source.isnumeric() or source.endswith('.txt') or source.lower().startswith(
-        ('rtsp://', 'rtmp://', 'http://', 'https://'))
+    is_file = Path(source).suffix[1:] in (IMG_FORMATS + VID_FORMATS)
+    is_url = source.lower().startswith(('rtsp://', 'rtmp://', 'http://', 'https://'))
+    webcam = source.isnumeric() or source.endswith('.txt') or (is_url and not is_file)
+    if is_url and is_file:
+        source = check_file(source)  # download
 
     # Directories
     save_dir = increment_path(Path(project) / name, exist_ok=exist_ok)  # increment run