`ImageFormat::from_mime_type` does not respect that MIME types are (mostly) case-insensitive

[RunDevelopment]

Quoting Wikipedia:

Types, subtypes, and parameter names are case-insensitive. Parameter values are usually case-sensitive, but may be interpreted in a case-insensitive fashion depending on the intended use.[6]

E.g. for text/plain;charset=UTF-8 everything but UTF-8 is case-insensitive and UTF-8 might be treated case-insensitive.

Unfortunately, ImageFormat::from_mime_type does not respect this and uses a case-sensitive comparison.

image/src/io/format.rs

Lines 150 to 157 in 4c05148

	pub fn from_mime_type<M>(mime_type: M) -> Option<Self>
	where
	M: AsRef<str>,
	{
	match mime_type.as_ref() {
	"image/avif" => Some(ImageFormat::Avif),
	"image/jpeg" => Some(ImageFormat::Jpeg),
	"image/png" => Some(ImageFormat::Png),

It would probably be easiest to just lower-case everything and treat everything as case-insensitive.

Expected

assert_eq!(ImageFormat::from_mime_type("IMAGE/PNG"), Some(ImageFormat::Png));

Actual behaviour

assert_eq!(ImageFormat::from_mime_type("IMAGE/PNG"), None);

[197g]

We should really use a proper mime library, yeah.

[fintelia]

Pulling in a whole library for dealing with mime types feels like scope creep. I'd be fine switching the checks to use str::eq_ignore_ascii_case, but anything beyond that I'd rather just document as a limitation. Any users that need fancy handling can always find a dedicated mime type library and/or implement it themselves.

[197g]

I expect there are some implementations that are lightweight, and it is a private dependency (looking at mediatype but it brings in tables of consts; would need to know if they get compiled out). We could roll our own of course but why. Note in the example text/plain;charset=UTF-8 that we really might deal with parameters as well (if only by removing them). tiff, svg, and avif/heif define optional parameters in their IANA registrations, it would not in principle be unexpected that parameters are being passed here.

[RunDevelopment]

we really might deal with parameters as well (if only by removing them)

Or we limit from_mime_type to only support mime types without parameters and document that users have to remove them. Maybe like this:

This function only supports MIME types without parameters and will return None for any MIME type with parameters.