GLTFLoader: Set RGBFormat for jpg with no mimeType.

[mrdoob]

Description

It seems like it can be the case that an image source has uri but not mimeType.
TextureLoader automatically sets RGBFormat based on the url but ImageBitmapLoader doesn't do that.

An example of a model is DamagedHelmet:

three.js/examples/models/gltf/DamagedHelmet/glTF/DamagedHelmet.gltf

Lines 98 to 114 in c3dd482

	"images" : [
	{
	"uri" : "Default_albedo.jpg"
	},
	{
	"uri" : "Default_metalRoughness.jpg"
	},
	{
	"uri" : "Default_emissive.jpg"
	},
	{
	"uri" : "Default_AO.jpg"
	},
	{
	"uri" : "Default_normal.jpg"
	}
	],

@donmccurdy @takahirox does this make sense to you?

[donmccurdy]

Yes, glTF only requires the mimeType field when the image is embedded as binary data (i.e. in a .glb). One other case that might hit a similar issue would be a Data URI, so perhaps the regexp should check for the data:image/jpeg; prefix at the beginning of the URI as well? Here's an example (although it uses PNG) ...

https://github.com/KhronosGroup/glTF-Sample-Models/blob/a334d137c0e86fa9455a0d6037d3aa8859af7abd/2.0/BoxTextured/glTF-Embedded/BoxTextured.gltf#L140-L144

[mrdoob]

@donmccurdy Done! 👍