Add support for Retrieve a file upload API

Src/Notion.Client/Api/ApiEndpoints.cs

@@ -146,6 +146,7 @@ public static class FileUploadsApiUrls
             public static string Send(string fileUploadId) => $"/v1/file_uploads/{fileUploadId}/send";
             public static string Complete(string fileUploadId) => $"/v1/file_uploads/{fileUploadId}/complete";
             public static string List => "/v1/file_uploads";
+            public static string Retrieve(IRetrieveFileUploadPathParameters pathParameters) => $"/v1/file_uploads/{pathParameters.FileUploadId}";
         }
     }
 }

Src/Notion.Client/Api/FileUploads/IFileUploadsClient.cs

@@ -51,5 +51,16 @@ Task<ListFileUploadsResponse> ListAsync(
             ListFileUploadsRequest request,
             CancellationToken cancellationToken = default
         );
+
+        /// <summary>
+        /// Retrieve a specific File Upload by its ID.
+        /// </summary>
+        /// <param name="request"></param>
+        /// <param name="cancellationToken"></param>
+        /// <returns></returns>
+        Task<RetrieveFileUploadResponse> RetrieveAsync(
+            RetrieveFileUploadRequest request,
+            CancellationToken cancellationToken = default
+        );
     }
 }

Src/Notion.Client/Api/FileUploads/Retrieve/FileUploadsClient.cs

@@ -0,0 +1,28 @@
+using System;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace Notion.Client
+{
+    public sealed partial class FileUploadsClient
+    {
+        public async Task<RetrieveFileUploadResponse> RetrieveAsync(
+            RetrieveFileUploadRequest request,
+            CancellationToken cancellationToken = default)
+        {
+            IRetrieveFileUploadPathParameters pathParameters = request;
+
+            if (pathParameters == null || string.IsNullOrEmpty(pathParameters.FileUploadId))
+            {
+                throw new ArgumentNullException(nameof(pathParameters.FileUploadId), "FileUploadId cannot be null or empty.");
+            }
+
+            var endpoint = ApiEndpoints.FileUploadsApiUrls.Retrieve(pathParameters);
+
+            return await _restClient.GetAsync<RetrieveFileUploadResponse>(
+                endpoint,
+                cancellationToken: cancellationToken
+            );
+        }
+    }
+}

Src/Notion.Client/Api/FileUploads/Retrieve/Request/IRetrieveFileUploadQueryParameters.cs

@@ -0,0 +1,10 @@
+namespace Notion.Client
+{
+    public interface IRetrieveFileUploadPathParameters
+    {
+        /// <summary>
+        /// The ID of the File Upload to retrieve.
+        /// </summary>
+        string FileUploadId { get; set; }
+    }
+}

Src/Notion.Client/Api/FileUploads/Retrieve/Request/RetrieveFileUploadRequest.cs

@@ -0,0 +1,7 @@
+namespace Notion.Client
+{
+    public class RetrieveFileUploadRequest : IRetrieveFileUploadPathParameters
+    {
+        public string FileUploadId { get; set; }
+    }
+}

Src/Notion.Client/Api/FileUploads/Retrieve/Response/RetrieveFileUploadResponse.cs

@@ -0,0 +1,6 @@
+namespace Notion.Client
+{
+    public class RetrieveFileUploadResponse : FileObjectResponse
+    {
+    }
+}

Test/Notion.IntegrationTests/FileUploadsClientTests.cs

@@ -139,5 +139,41 @@ public async Task ListAsync()
             Assert.NotNull(response.Results);
             Assert.True(response.Results.Count <= 5);
         }
+
+        [Fact]
+        public async Task RetrieveAsync()
+        {
+            // Arrange
+            var createRequest = new CreateFileUploadRequest
+            {
+                Mode = FileUploadMode.ExternalUrl,
+                ExternalUrl = "https://unsplash.com/photos/hOhlYhAiizc/download?ixid=M3wxMjA3fDB8MXxhbGx8fHx8fHx8fHwxNzYwMTkxNzc3fA&force=true",
+                FileName = "sample-image.jpg",
+            };
+
+            var createResponse = await Client.FileUploads.CreateAsync(createRequest);
+
+            Assert.NotNull(createResponse);
+            Assert.NotNull(createResponse.Id);
+            Assert.Equal("sample-image.jpg", createResponse.FileName);
+            Assert.Equal("image/jpeg", createResponse.ContentType);
+            Assert.Equal("pending", createResponse.Status);
+
+            // Act
+            var retrieveRequest = new RetrieveFileUploadRequest
+            {
+                FileUploadId = createResponse.Id
+            };
+
+            var retrieveResponse = await Client.FileUploads.RetrieveAsync(retrieveRequest);
+
+            // Assert
+            Assert.NotNull(retrieveResponse);
+            Assert.Equal(createResponse.Id, retrieveResponse.Id);
+            Assert.Equal("sample-image.jpg", retrieveResponse.FileName);
+            Assert.Equal("image/jpeg", retrieveResponse.ContentType);
+            // The status might have changed from "pending" to "uploaded" depending on Notion's processing time
+            Assert.Contains(retrieveResponse.Status, new[] { "pending", "uploaded" });
+        }
     }
-}
+}

Test/Notion.UnitTests/FileUploadsClientTests.cs

@@ -219,4 +219,61 @@ public async Task ListAsync_CallsRestClientGetAsync_WithCorrectParameters()
     }
 
     #endregion ListAsync Tests
+
+    #region RetrieveAsync Tests
+
+    // Add tests for RetrieveAsync method
+    [Fact]
+    public async Task RetrieveAsync_ThrowsArgumentNullException_WhenFileUploadIdIsNullOrEmpty()
+    {
+        // Arrange
+        var request = new RetrieveFileUploadRequest
+        {
+            FileUploadId = null
+        };
+
+        // Act & Assert
+        var exception = await Assert.ThrowsAsync<ArgumentNullException>(() => _fileUploadClient.RetrieveAsync(request));
+        Assert.Equal("FileUploadId", exception.ParamName);
+        Assert.Equal("FileUploadId cannot be null or empty. (Parameter 'FileUploadId')", exception.Message);
+    }
+
+    [Fact]
+    public async Task RetrieveAsync_CallsRestClientGetAsync_WithCorrectParameters()
+    {
+        // Arrange
+        var fileUploadId = Guid.NewGuid().ToString();
+        var request = new RetrieveFileUploadRequest
+        {
+            FileUploadId = fileUploadId
+        };
+
+        var expectedResponse = new RetrieveFileUploadResponse
+        {
+            Id = fileUploadId,
+            FileName = "testfile.txt",
+            Status = "completed"
+        };
+
+        _restClientMock
+            .Setup(client => client.GetAsync<RetrieveFileUploadResponse>(
+                It.Is<string>(url => url == ApiEndpoints.FileUploadsApiUrls.Retrieve(request)),
+                It.IsAny<IDictionary<string, string>>(),
+                null,
+                null,
+                It.IsAny<CancellationToken>()
+            ))
+            .ReturnsAsync(expectedResponse);
+
+        // Act
+        var response = await _fileUploadClient.RetrieveAsync(request);
+
+        // Assert
+        Assert.Equal(expectedResponse.Id, response.Id);
+        Assert.Equal(expectedResponse.FileName, response.FileName);
+        Assert.Equal(expectedResponse.Status, response.Status);
+        _restClientMock.VerifyAll();
+    }
+
+    #endregion RetrieveAsync Tests
 }