fix(useFilenamingConvention): apply formats to file extensions

CHANGELOG.md

@@ -40,11 +40,23 @@ our [guidelines for writing a good changelog entry](https://github.com/biomejs/b
 
 - Add [nursery/noMissingVarFunction](https://biomejs.dev/linter/rules/no-missing-var-function). Contributed by @michellocana
 
+#### Bug fixes
+
+- [useFilenamingConvention](https://biomejs.dev/linter/rules/use-filenaming-convention) no longer suggests names with a disallowed case ([#3952](https://github.com/biomejs/biome/issues/3952)). Contributed by @Conaclos
+
+- [useFilenamingConvention](https://biomejs.dev/linter/rules/use-filenaming-convention) now recognizes file names starting with ASCII digits as lowercase ([#3952](https://github.com/biomejs/biome/issues/3952)).
+
+  Thus, `2024-09-17-filename`, `2024_09_17_filename` and `20240917FileName` are in `kebab-case`, `snake_case`, and `camelCase` respectively.
+
+  Contributed by @Conaclos
+
+- [useFilenamingConvention](https://biomejs.dev/linter/rules/use-filenaming-convention) now applies the configured formats to the file extensions ([#3650](https://github.com/biomejs/biome/discussions/3650)). Contributed by @Conaclos
+
 ### Parser
 
 #### Bug fixes
 
-- [useStrictMode](https://biomejs.dev/linter/rules/use-strict-mode/) now reports Script files with dome diretcives, but without the `use strict` directive. Contributed by @Conaclos
+- [useStrictMode](https://biomejs.dev/linter/rules/use-strict-mode/) now reports Script files with some directives, but without the `use strict` directive. Contributed by @Conaclos
 
 - The CSS parser now accepts the characters U+FFDCF and U+FFFD in identifiers. Contributed by @Conaclos
 
@@ -70,14 +82,6 @@ our [guidelines for writing a good changelog entry](https://github.com/biomejs/b
 
 #### Bug fixes
 
-- [useFilenamingConvention](https://biomejs.dev/linter/rules/use-filenaming-convention) no longer suggests names with a disallowed case ([#3952](https://github.com/biomejs/biome/issues/3952)). Contributed by @Conaclos
-
-- [useFilenamingConvention](https://biomejs.dev/linter/rules/use-filenaming-convention) now recognizes file names starting with ASCII digits as lowercase ([#3952](https://github.com/biomejs/biome/issues/3952)).
-
-  Thus, `2024-09-17-filename`, `2024_09_17_filename` and `20240917FileName` are in `kebab-case`, `snake_case`, and `camelCase` respectively.
-
-  Contributed by @Conaclos
-
 - [useSemanticElements](https://biomejs.dev/linter/rules/use-semantic-elements/) now ignores `alert` and `alertdialog` roles ([3858](https://github.com/biomejs/biome/issues/3858)). Contributed by @Conaclos
 
 - [noUndeclaredDependencies](https://biomejs.dev/linter/rules/no-undeclared-dependencies/) now ignores `@/` imports and recognizes type imports from Definitely Typed and `bun` imports. Contributed by @Conaclos

crates/biome_js_analyze/src/lint/style/use_filenaming_convention.rs

@@ -34,6 +34,7 @@ declare_lint_rule! {
     ///
     /// By default, the rule ensures that the filename is either in [`camelCase`], [`kebab-case`], [`snake_case`],
     /// or equal to the name of one export in the file.
+    /// By default, the rule ensures that the extensions are either in [`camelCase`], [`kebab-case`], or [`snake_case`].
     ///
     /// ## Ignoring some files
     ///
@@ -104,6 +105,9 @@ declare_lint_rule! {
     /// You can enforce a stricter convention by setting `filenameCases` option.
     /// `filenameCases` accepts an array of cases among the following cases: [`camelCase`], [`kebab-case`], [`PascalCase`], [`snake_case`], and `export`.
     ///
+    /// This option also applies to the file extensions.
+    /// Extensions in lowercase are always allowed regardless of how `filenameCases` is set.
+    ///
     /// [case]: https://en.wikipedia.org/wiki/Naming_convention_(programming)#Examples_of_multiple-word_identifier_formats
     /// [`camelCase`]: https://en.wikipedia.org/wiki/Camel_case
     /// [`kebab-case`]: https://en.wikipedia.org/wiki/Letter_case#Kebab_case
@@ -185,15 +189,18 @@ impl Rule for UseFilenamingConvention {
             };
             (name, split)
         };
+        let allowed_cases = options.filename_cases.cases();
+        let allowed_extension_cases = allowed_cases | Case::Lower;
         // Check extension case
-        if extensions.any(|extension| Case::identify(extension, true) != Case::Lower) {
+        if extensions.any(|extension| {
+            !allowed_extension_cases.contains(Case::identify(extension, options.strict_case))
+        }) {
             return Some(FileNamingConventionState::Extension);
         }
         if name.is_empty() {
             return None;
         }
         // Check filename case
-        let allowed_cases = options.filename_cases.cases();
         if !allowed_cases.is_empty() {
             let trimmed_name = name.trim_matches('_');
             let case = Case::identify(trimmed_name, options.strict_case);
@@ -241,7 +248,7 @@ impl Rule for UseFilenamingConvention {
                         .join(" or ")
                 } else {
                     allowed_case_names
-                        .collect::<SmallVec<[_; 3]>>()
+                        .collect::<SmallVec<[_; 4]>>()
                         .join(" or ")
                 };
                 let mut split = file_name.split('.');
@@ -306,11 +313,14 @@ impl Rule for UseFilenamingConvention {
                 }))
             },
             FileNamingConventionState::Extension => {
+                let allowed_cases = options.filename_cases.cases() | Case::Lower;
+                let allowed_case_names = allowed_cases.into_iter().map(|case| case.to_string());
+                let allowed_case_names = allowed_case_names.collect::<SmallVec<[_; 4]>>().join(" or ");
                 Some(RuleDiagnostic::new(
                     rule_category!(),
                     None as Option<TextRange>,
                     markup! {
-                        "The file extension should be in lowercase without any special characters."
+                        "The file extension should be in "<Emphasis>{allowed_case_names}</Emphasis>"."
                     },
                 ))
             },

crates/biome_js_analyze/tests/specs/style/useFilenamingConvention/filename.invalid-extension.js.snap → crates/biome_js_analyze/tests/specs/style/useFilenamingConvention/filename.INVALID-extension.js.snap

@@ -1,6 +1,6 @@
 ---
 source: crates/biome_js_analyze/tests/spec_tests.rs
-expression: filename.invalid-extension.js
+expression: filename.INVALID-extension.js
 ---
 # Input
 ```jsx
@@ -9,11 +9,9 @@ expression: filename.invalid-extension.js
 
 # Diagnostics
 ```
-filename.invalid-extension.js lint/style/useFilenamingConvention ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
+filename.INVALID-extension.js lint/style/useFilenamingConvention ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
 
-  ! The file extension should be in lowercase without any special characters.
+  ! The file extension should be in camelCase or kebab-case or snake_case.
 
 
 ```
-
-

crates/biome_js_analyze/tests/specs/style/useFilenamingConvention/filename.INVALID.js.snap

@@ -0,0 +1,17 @@
+---
+source: crates/biome_js_analyze/tests/spec_tests.rs
+expression: filename.INVALID.js
+---
+# Input
+```jsx
+
+```
+
+# Diagnostics
+```
+filename.INVALID.js lint/style/useFilenamingConvention ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
+
+  ! The file extension should be in lowercase.
+
+
+```

crates/biome_js_analyze/tests/specs/style/useFilenamingConvention/filename.INVALID.options.json

@@ -0,0 +1,15 @@
+{
+	"$schema": "../../../../../../packages/@biomejs/biome/configuration_schema.json",
+	"linter": {
+		"rules": {
+			"style": {
+				"useFilenamingConvention": {
+					"level": "error",
+					"options": {
+						"filenameCases": ["export"]
+					}
+				}
+			}
+		}
+	}
+}

crates/biome_js_analyze/tests/specs/style/useFilenamingConvention/filename.valid-kebab-extension.js.snap

@@ -0,0 +1,8 @@
+---
+source: crates/biome_js_analyze/tests/spec_tests.rs
+expression: filename.valid-kebab-extension.js
+---
+# Input
+```jsx
+
+```

crates/biome_js_analyze/tests/specs/style/useFilenamingConvention/filename.validCamelExt.js.snap

@@ -0,0 +1,8 @@
+---
+source: crates/biome_js_analyze/tests/spec_tests.rs
+expression: filename.validCamelExt.js
+---
+# Input
+```jsx
+
+```

crates/biome_js_analyze/tests/specs/style/useFilenamingConvention/filename.valid_snake_ext.js.snap

@@ -0,0 +1,8 @@
+---
+source: crates/biome_js_analyze/tests/spec_tests.rs
+expression: filename.valid_snake_ext.js
+---
+# Input
+```jsx
+
+```