Clarify cross_file MIME type docs

Author: mackings

packages/cross_file/CHANGELOG.md

@@ -1,5 +1,6 @@
 ## NEXT
 
+* Clarifies that `XFile.mimeType` is not inferred from the file path or contents.
 * Updates minimum supported SDK version to Flutter 3.35/Dart 3.9.
 
 ## 0.3.5+2

packages/cross_file/README.md

@@ -23,6 +23,10 @@ final String fileContent = await file.readAsString();
 print('Content of the file: $fileContent');
 ```
 
+The `mimeType` property is not inferred from the file path or contents. It is
+only available if provided when the `XFile` was created, or if available from the
+platform-backed file.
+
 You will find links to the API docs on the [pub page](https://pub.dev/packages/cross_file).
 
 ## Web Limitations

packages/cross_file/lib/src/types/base.dart

@@ -45,7 +45,11 @@ abstract class XFileBase {
     throw UnimplementedError('.name has not been implemented.');
   }
 
-  /// For web, it may be necessary for a file to know its MIME type.
+  /// The MIME type of the file.
+  ///
+  /// The value is not inferred from the file contents or path. It is only
+  /// available if provided when the [XFile] was created, or if available from
+  /// the platform-backed file.
   String? get mimeType {
     throw UnimplementedError('.mimeType has not been implemented.');
   }

packages/cross_file/lib/src/types/html.dart

@@ -26,6 +26,8 @@ class XFile extends XFileBase {
   ///
   /// `name` needs to be passed from the outside, since it's only available
   /// while handling [html.File]s (when the ObjectUrl is created).
+  ///
+  /// [mimeType] is only available if explicitly passed to the constructor.
   // ignore: use_super_parameters
   XFile(
     String path, {
@@ -47,6 +49,8 @@ class XFile extends XFileBase {
   }
 
   /// Construct an CrossFile from its data
+  ///
+  /// [mimeType] is only available if explicitly passed to the constructor.
   XFile.fromData(
     Uint8List bytes, {
     String? mimeType,

packages/cross_file/lib/src/types/io.dart

@@ -24,6 +24,9 @@ class XFile extends XFileBase {
   /// [length] is ignored; the parameter exists only to match the web version of
   /// the constructor.
   ///
+  /// [mimeType] is not inferred from [path] and is only available if explicitly
+  /// passed to the constructor.
+  ///
   // ignore: use_super_parameters
   XFile(
     String path, {
@@ -40,6 +43,8 @@ class XFile extends XFileBase {
 
   /// Construct an CrossFile from its data
   ///
+  /// [mimeType] is only available if explicitly passed to the constructor.
+  ///
   /// [name] is ignored; the parameter exists only to match the web version of
   /// the constructor.
   XFile.fromData(