Improve signature ordering detection requirements (#518)

Co-authored-by: Sindre Sorhus <sindresorhus@gmail.com>

.github/pull_request_template.md

@@ -4,7 +4,11 @@ If you're adding support for a new file type, please follow the below steps:
 - Add a fixture file named `fixture.<extension>` to the `fixture` directory.
 - Add the file extension to the `extensions` array in `supported.js`.
 - Add the file's MIME type to the `types` array in `supported.js`.
-- Add the file type detection logic to the `core.js` file.
+- Add the file type detection logic to the `core.js` file
+- Respect the sequence:
+	- Signature with shorter sample size (counted from offset 0 until the last required byte position) will be executed first.
+	- Only the initial determination for the file type counts for the sequence.
+	- Existing signatures requiring same sample length (same *signature group*) will be tested prior to your new detections. Yours will be last. (rational: common formats first).
 - Add the file extension to the `FileType` type in `core.d.ts`.
 - Add the file's MIME type to the `MimeType` type in `core.d.ts`.
 - Add the file extension to the `Supported file types` section in the readme, in the format ```- [`<extension>`](URL) - Format name```, for example, ```- [`png`](https://en.wikipedia.org/wiki/Portable_Network_Graphics) - Portable Network Graphics```

core.js

@@ -151,6 +151,13 @@ class FileTypeParser {
 
 		// -- 3-byte signatures --
 
+		if (this.check([0x47, 0x49, 0x46])) {
+			return {
+				ext: 'gif',
+				mime: 'image/gif',
+			};
+		}
+
 		if (this.check([0xFF, 0xD8, 0xFF])) {
 			return {
 				ext: 'jpg',
@@ -214,20 +221,6 @@ class FileTypeParser {
 
 		// -- 4-byte signatures --
 
-		if (this.check([0x7F, 0x45, 0x4C, 0x46])) {
-			return {
-				ext: 'elf',
-				mime: 'application/x-elf',
-			};
-		}
-
-		if (this.check([0x47, 0x49, 0x46])) {
-			return {
-				ext: 'gif',
-				mime: 'image/gif',
-			};
-		}
-
 		if (this.checkString('FLIF')) {
 			return {
 				ext: 'flif',
@@ -798,6 +791,13 @@ class FileTypeParser {
 			};
 		}
 
+		if (this.check([0x7F, 0x45, 0x4C, 0x46])) {
+			return {
+				ext: 'elf',
+				mime: 'application/x-elf',
+			};
+		}
+
 		// -- 5-byte signatures --
 
 		if (this.check([0x4F, 0x54, 0x54, 0x4F, 0x00])) {