Add support for .txt files (#27)

Author: detilium

src/ByteGuard.FileValidator/FileExtensions.cs

@@ -109,5 +109,10 @@ public static class FileExtensions
         /// Waveform Audio File Format.
         /// </summary>
         public const string Wav = ".wav";
+
+        /// <summary>
+        /// Text File.
+        /// </summary>
+        public const string Txt = ".txt";
     }
 }

src/ByteGuard.FileValidator/FileValidator.cs

@@ -230,6 +230,11 @@ public class FileValidator
                 {
                     new byte[] { 0x57, 0x41, 0x56, 0x45 } // WAVE
                 }
+            },
+            new FileDefinition
+            {
+                FileType = FileExtensions.Txt,
+                AllowMissingSignature = true
             }
         };
 
@@ -522,6 +527,12 @@ public bool HasValidSignature(string fileName, Stream stream)
                 return false;
             }
 
+            // If the file definition allows for missing signatures, we can return early as the signature validation is effectively bypassed.
+            if (fileDefinition.AllowMissingSignature)
+            {
+                return true;
+            }
+
             // As PDF documents are somewhat special in terms of both signature validation,
             // we need to investigate these files further. The PdfValidator is made specifically
             // for this purpose.

src/ByteGuard.FileValidator/Models/FileDefinition.cs

@@ -10,6 +10,16 @@ internal class FileDefinition
         /// </summary>
         public string FileType { get; set; } = default!;
 
+        /// <summary>
+        /// Whether this file type is allowed to omit a binary signature. Defaults to <c>false</c>.
+        /// </summary>
+        /// <remarks>
+        /// Some file types, such as .txt files, do not have a binary signature.
+        /// Setting this property to <c>true</c> allows files of this type to be considered valid even if they do not have a binary signature.
+        /// This is useful for file types that do not have a binary signature, but still need to be validated by other mechanisms.
+        /// </remarks>
+        public bool AllowMissingSignature { get; set; } = false;
+
         /// <summary>
         /// Valid header signatures.
         /// </summary>

tests/ByteGuard.FileValidator.Tests.Unit/FileValidatorTests.cs

@@ -418,6 +418,26 @@ public void HasValidSignature_FileNameDoesNotHaveExtension_ShouldThrowArgumentEx
         Assert.Throws<ArgumentException>(act);
     }
 
+    [Theory(DisplayName = "HasValidSignature(byte[]) should skip signature validation for file types that allow missing signatures and return true")]
+    [InlineData(new byte[] { 0x74, 0x65, 0x73, 0x74 }, "test.txt")] // TXT
+    public void HasValidSignature_FileTypeAllowsMissingSignature_ShouldReturnTrue(byte[] fileBytes, string fileName)
+    {
+        // Arrange
+        var config = new FileValidatorConfiguration
+        {
+            SupportedFileTypes = [Path.GetExtension(fileName)],
+            FileSizeLimit = ByteSize.MegaBytes(25),
+            ThrowExceptionOnInvalidFile = true
+        };
+        var fileValidator = new FileValidator(config);
+
+        // Act
+        var result = fileValidator.HasValidSignature(fileName, fileBytes);
+
+        // Assert
+        Assert.True(result);
+    }
+
     [Theory(DisplayName = "IsOpenXmlFormat should return true for valid Open XML files")]
     [InlineData("test.docx")] // DOCX
     [InlineData("test.xlsx")] // XLSX