feat: add files endpoint with info, download, upload, and delete

Implement FileInfo, FileResponse, SaveFileRequest, and
CreatedWithIdResponse models. Add get_file_info, get_file (binary
download), upload_file (multipart), and delete_file methods. Tests
cover metadata retrieval, binary content download with headers, file
upload, deletion, and not-found error with message extraction.

src/endpoints/files.rs

@@ -0,0 +1,88 @@
+use reqwest::Method;
+
+use crate::client::OnspringClient;
+use crate::error::Result;
+use crate::models::{CreatedWithIdResponse, FileInfo, FileResponse, SaveFileRequest};
+
+impl OnspringClient {
+    /// Gets a file's metadata information.
+    pub async fn get_file_info(
+        &self,
+        record_id: i32,
+        field_id: i32,
+        file_id: i32,
+    ) -> Result<FileInfo> {
+        let path = format!(
+            "/Files/recordId/{}/fieldId/{}/fileId/{}",
+            record_id, field_id, file_id
+        );
+        self.request(Method::GET, &path, &[], Option::<&()>::None)
+            .await
+    }
+
+    /// Downloads a file's content.
+    pub async fn get_file(
+        &self,
+        record_id: i32,
+        field_id: i32,
+        file_id: i32,
+    ) -> Result<FileResponse> {
+        let path = format!(
+            "/Files/recordId/{}/fieldId/{}/fileId/{}/file",
+            record_id, field_id, file_id
+        );
+        let (_status, headers, data) = self.request_bytes(Method::GET, &path, &[]).await?;
+
+        let content_type = headers
+            .get("content-type")
+            .and_then(|v| v.to_str().ok())
+            .map(String::from);
+
+        let file_name = headers
+            .get("content-disposition")
+            .and_then(|v| v.to_str().ok())
+            .and_then(|v| {
+                v.split("filename=")
+                    .nth(1)
+                    .map(|s| s.trim_matches('"').to_string())
+            });
+
+        Ok(FileResponse {
+            content_type,
+            file_name,
+            data,
+        })
+    }
+
+    /// Uploads a file attachment.
+    pub async fn upload_file(&self, request: SaveFileRequest) -> Result<CreatedWithIdResponse> {
+        let file_part = reqwest::multipart::Part::bytes(request.file_data)
+            .file_name(request.file_name)
+            .mime_str(&request.content_type)
+            .map_err(|e| crate::error::OnspringError::InvalidArgument(e.to_string()))?;
+
+        let mut form = reqwest::multipart::Form::new()
+            .text("RecordId", request.record_id.to_string())
+            .text("FieldId", request.field_id.to_string())
+            .part("File", file_part);
+
+        if let Some(notes) = request.notes {
+            form = form.text("Notes", notes);
+        }
+        if let Some(date) = request.modified_date {
+            form = form.text("ModifiedDate", date.to_rfc3339());
+        }
+
+        self.request_multipart("/Files", form).await
+    }
+
+    /// Deletes a file attachment.
+    pub async fn delete_file(&self, record_id: i32, field_id: i32, file_id: i32) -> Result<()> {
+        let path = format!(
+            "/Files/recordId/{}/fieldId/{}/fileId/{}",
+            record_id, field_id, file_id
+        );
+        self.request_no_content(Method::DELETE, &path, &[], Option::<&()>::None)
+            .await
+    }
+}

src/endpoints/mod.rs

@@ -1,4 +1,5 @@
 mod apps;
 mod fields;
+mod files;
 mod ping;
 mod records;

src/models/file.rs

@@ -0,0 +1,45 @@
+use bytes::Bytes;
+use chrono::{DateTime, Utc};
+use serde::Deserialize;
+
+/// Information about a file attachment.
+#[derive(Debug, Clone, Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub struct FileInfo {
+    #[serde(rename = "type")]
+    pub file_type: Option<String>,
+    pub content_type: Option<String>,
+    pub name: Option<String>,
+    pub created_date: Option<DateTime<Utc>>,
+    pub modified_date: Option<DateTime<Utc>>,
+    pub owner: Option<String>,
+    pub notes: Option<String>,
+    pub file_href: Option<String>,
+}
+
+/// The content of a downloaded file.
+#[derive(Debug, Clone)]
+pub struct FileResponse {
+    pub content_type: Option<String>,
+    pub file_name: Option<String>,
+    pub data: Bytes,
+}
+
+/// Request to upload a file.
+#[derive(Debug, Clone)]
+pub struct SaveFileRequest {
+    pub record_id: i32,
+    pub field_id: i32,
+    pub notes: Option<String>,
+    pub modified_date: Option<DateTime<Utc>>,
+    pub file_name: String,
+    pub file_data: Vec<u8>,
+    pub content_type: String,
+}
+
+/// Response from creating a file, containing the new file ID.
+#[derive(Debug, Clone, Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub struct CreatedWithIdResponse {
+    pub id: i32,
+}

src/models/mod.rs

@@ -1,11 +1,13 @@
 mod app;
 mod enums;
 mod field;
+pub mod file;
 mod paging;
 pub mod record;
 
 pub use app::*;
 pub use enums::*;
 pub use field::*;
+pub use file::*;
 pub use paging::*;
 pub use record::*;