Initialize Fields if null when adding attachment metadata field

See #1826
Fix issue where FileField was mapped as "file" and not "content"
Fix issue where Language fluent method accepted a NumberPropertyDescriptor<T> instead of StringPropertyDescriptor<T>
Fix issue where ContentLength fluent method accepted a StringPropertyDescriptor<T> instead of NumberPropertyDescriptor<T>
Both of these fluent method changes overload the fluent method and mean that the lambda expression must be typed

Author: russcam

Diff for: src/Nest/Mapping/Types/Specialized/Attachment/AttachmentProperty.cs

@@ -1,5 +1,5 @@
 using System;
-using System.Collections;
+using System.Collections.Generic;
 using Newtonsoft.Json;
 
 namespace Nest
@@ -22,7 +22,7 @@ public class AttachmentProperty : PropertyBase, IAttachmentProperty
 	{
 		public AttachmentProperty() : base("attachment") { }
 
-		private IDictionary Dictionary => this.Fields;
+		private IDictionary<PropertyName, IProperty> Dictionary => this.Fields ?? (this.Fields = new Properties());
 
 		public IStringProperty AuthorField
 		{
@@ -50,8 +50,8 @@ public IDateProperty DateField
 
 		public IStringProperty FileField
 		{
-			get { return Dictionary["file"] as IStringProperty; }
-			set { Dictionary["file"] = value; }
+			get { return Dictionary["content"] as IStringProperty; }
+			set { Dictionary["content"] = value; }
 		}
 
 		public IStringProperty KeywordsField
@@ -93,7 +93,7 @@ public class AttachmentPropertyDescriptor<T>
 		INumberProperty IAttachmentProperty.ContentLengthField { get; set; }
 		IStringProperty IAttachmentProperty.LanguageField { get; set; }
 
-		private IDictionary Dictionary => Self.Fields;
+		private IDictionary<PropertyName, IProperty> Dictionary => Self.Fields ?? (Self.Fields = new Properties());
 
 		public AttachmentPropertyDescriptor() : base("attachment") { }
 
@@ -117,7 +117,7 @@ public AttachmentPropertyDescriptor<T> NameField(Func<StringPropertyDescriptor<T
 			SetMetadataField(selector, "name");
 
 		public AttachmentPropertyDescriptor<T> FileField(Func<StringPropertyDescriptor<T>, IStringProperty> selector) =>
-			SetMetadataField(selector, "file");
+			SetMetadataField(selector, "content");
 
 		public AttachmentPropertyDescriptor<T> AuthorField(Func<StringPropertyDescriptor<T>, IStringProperty> selector) =>
 			SetMetadataField(selector, "author");
@@ -128,10 +128,18 @@ public AttachmentPropertyDescriptor<T> KeywordsField(Func<StringPropertyDescript
 		public AttachmentPropertyDescriptor<T> ContentTypeField(Func<StringPropertyDescriptor<T>, IStringProperty> selector) =>
 			SetMetadataField(selector, "content_type");
 
+		[Obsolete("Use ContentLengthField(Func<NumberPropertyDescriptor<T>, INumberProperty> selector)")]
 		public AttachmentPropertyDescriptor<T> ContentLengthField(Func<StringPropertyDescriptor<T>, IStringProperty> selector) =>
 			SetMetadataField(selector, "content_length");
 
+		public AttachmentPropertyDescriptor<T> ContentLengthField(Func<NumberPropertyDescriptor<T>, INumberProperty> selector) =>
+			SetMetadataField(selector, "content_length");
+
+		[Obsolete("Use LanguageField(Func<StringPropertyDescriptor<T>, IStringProperty> selector)")]
 		public AttachmentPropertyDescriptor<T> LanguageField(Func<NumberPropertyDescriptor<T>, INumberProperty> selector) =>
 			SetMetadataField(selector, "language");
+
+		public AttachmentPropertyDescriptor<T> LanguageField(Func<StringPropertyDescriptor<T>, IStringProperty> selector) =>
+			SetMetadataField(selector, "language");
 	}
 }

Diff for: src/Tests/Mapping/Types/Specialized/Attachment/AttachmentMappingTests.cs

@@ -0,0 +1,126 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading.Tasks;
+using Nest;
+using Xunit;
+
+namespace Tests.Mapping.Types.Specialized.Attachment
+{
+	public class AttachmentTest
+	{
+		public string File { get; set; }
+
+		public string Author { get; set; }
+
+		public long ContentLength { get; set; }
+
+		public string ContentType { get; set; }
+
+		public DateTime Date { get; set; }
+
+		public string Keywords { get; set; }
+
+		public string Language { get; set; }
+
+		public string Name { get; set; }
+
+		public string Title { get; set; }
+	}
+
+	public class AttachmentMappingTests : TypeMappingTestBase<AttachmentTest>
+	{
+		protected override object ExpectJson => new
+		{
+			properties = new
+			{
+				file = new
+				{
+					type = "attachment",
+					fields = new
+					{
+						author = new
+						{
+							type = "string"
+						},
+						content = new
+						{
+							type = "string"
+						},
+						content_length = new
+						{
+							type = "double"
+						},
+						content_type = new
+						{
+							type = "string"
+						},
+						date = new
+						{
+							type = "date"
+						},
+						keywords = new
+						{
+							type = "string"
+						},
+						language = new
+						{
+							type = "string",
+							doc_values = true,
+							index = "not_analyzed"
+						},
+						name = new
+						{
+							type = "string"
+						},
+						title = new
+						{
+							type = "string"
+						}
+					}
+				}
+			}
+		};
+
+		protected override void AttributeBasedSerializes()
+		{
+			// TODO: Implement
+		}
+
+		protected override Func<PropertiesDescriptor<AttachmentTest>, IPromise<IProperties>> FluentProperties => p => p
+			.Attachment(a => a
+				//.Fields(s => s)
+				.Name(n => n.File)
+				.AuthorField(d => d
+					.Name(n => n.Author)
+				)
+				.FileField(d => d
+					.Name(n => n.File)
+				)
+				.ContentLengthField((NumberPropertyDescriptor<AttachmentTest> d) => d
+					.Name(n => n.ContentLength)
+				)
+				.ContentTypeField(d => d
+					.Name(n => n.ContentType)
+				)
+				.DateField(d => d
+					.Name(n => n.Date)
+				)
+				.KeywordsField(d => d
+					.Name(n => n.Keywords)
+				)
+				.LanguageField((StringPropertyDescriptor<AttachmentTest> d) => d
+					.Name(n => n.Language)
+					.DocValues()
+					.NotAnalyzed()
+				)
+				.NameField(d => d
+					.Name(n => n.Name)
+				)
+				.TitleField(d => d
+					.Name(n => n.Title)
+				)
+			);
+	}
+}
+