Fix DataUriParser to default to text/plain;charset=US-ASCII per RFC 2397

src/Libraries/Microsoft.Extensions.AI.Abstractions/Contents/DataUriParser.cs

@@ -23,6 +23,11 @@ internal static class DataUriParser
 {
     public static string Scheme => "data:";
 
+    /// <summary>
+    /// The default media type per RFC 2397 when the media type is omitted.
+    /// </summary>
+    public const string DefaultMediaType = "text/plain;charset=US-ASCII";
+
     public static DataUri Parse(ReadOnlyMemory<char> dataUri)
     {
         // Validate, then trim off the "data:" scheme.
@@ -59,9 +64,14 @@ public static DataUri Parse(ReadOnlyMemory<char> dataUri)
         }
 
         // Validate the media type, if present.
+        // Per RFC 2397, if the media type is omitted, it defaults to "text/plain;charset=US-ASCII".
         ReadOnlySpan<char> span = metadata.Span.Trim();
         string? mediaType = null;
-        if (!span.IsEmpty && !IsValidMediaType(span, ref mediaType))
+        if (span.IsEmpty)
+        {
+            mediaType = DefaultMediaType;
+        }
+        else if (!IsValidMediaType(span, ref mediaType))
         {
             throw new UriFormatException("Invalid data URI format: the media type is not a valid.");
         }
@@ -91,6 +101,7 @@ public static bool IsValidMediaType(ReadOnlySpan<char> mediaTypeSpan, [NotNull]
         // For common media types, we can avoid both allocating a string for the span and avoid parsing overheads.
         string? knownType = mediaTypeSpan switch
         {
+            DefaultMediaType => DefaultMediaType,
             "application/json" => "application/json",
             "application/octet-stream" => "application/octet-stream",
             "application/pdf" => "application/pdf",

test/Libraries/Microsoft.Extensions.AI.Abstractions.Tests/Contents/DataContentTests.cs

@@ -109,6 +109,35 @@ public void Ctor_NoMediaType_Roundtrips()
         Assert.Equal("aGVsbG8=", content.Base64Data.ToString());
     }
 
+    [Theory]
+    [InlineData("data:,hello", "hello")]
+    [InlineData("data:;base64,aGVsbG8=", "hello")]
+    [InlineData("data:,hello%20world", "hello world")]
+    [InlineData("data:,", "")]
+    [InlineData("data:;base64,", "")]
+    public void Ctor_OmittedMediaType_DefaultsToTextPlain(string uri, string expectedData)
+    {
+        // Per RFC 2397, if the media type is omitted, it defaults to "text/plain;charset=US-ASCII"
+        static void Validate(DataContent content, string expectedData)
+        {
+            Assert.Equal("text/plain;charset=US-ASCII", content.MediaType);
+            Assert.Equal(expectedData, Encoding.UTF8.GetString(content.Data.ToArray()));
+        }
+
+        Validate(new DataContent(uri), expectedData);
+        Validate(new DataContent(new Uri(uri)), expectedData);
+    }
+
+    [Theory]
+    [InlineData("data:,hello", "application/json")]
+    [InlineData("data:;base64,aGVsbG8=", "application/octet-stream")]
+    public void Ctor_OmittedMediaType_CanBeOverridden(string uri, string mediaType)
+    {
+        // When media type is omitted in the URI but provided as a parameter, the parameter takes precedence
+        var content = new DataContent(uri, mediaType);
+        Assert.Equal(mediaType, content.MediaType);
+    }
+
     [Fact]
     public void Serialize_MatchesExpectedJson()
     {