added Api getFile and downloadFile methods

Use Api::getFile method to get basic information about a file and prepare it for downloading.
Use Api::downloadFile method to download a file from Telegram and save it in memory.

include/tgbotxx/Api.hpp

@@ -311,6 +311,25 @@ namespace tgbotxx {
                                 bool allowSendingWithoutReply = false,
                                 const Ptr<IReplyMarkup>& replyMarkup = nullptr) const;
 
+      /// @brief Use this method to get basic information about a file and prepare it for downloading.
+      /// For the moment, bots can download files of up to 20MB in size.
+      /// The file can then be downloaded using Api::downloadFile or via the link https://api.telegram.org/file/bot<token>/<file_path>, where <file_path> is taken from the response.
+      /// @param fileId File identifier to get information about
+      /// @returns a File object is returned on success.
+      /// @note It is guaranteed that the link will be valid for at least 1 hour. When the link expires, a new one can be requested by calling getFile again.
+      /// @note This function may not preserve the original file name and MIME type. You should save the file's MIME type and name (if available) when the File object is received.
+      /// @ref https://core.telegram.org/bots/api#getfile
+      Ptr<File> getFile(const std::string& fileId) const;
+
+      /// @brief Use this method to download a file from Telegram and save it in memory.
+      /// For the moment, bots can download files of up to 20MB in size. See Api::getFile.
+      /// The file can then be downloaded via the link https://api.telegram.org/file/bot<token>/<file_path>, where <file_path> is taken from the response.
+      /// @param filePath Telegram file path from Api::getFile(fileId) -> File::filePath
+      /// @returns std::string contains downloaded file contents.
+      /// @ref https://core.telegram.org/bots/api#getfile
+      /// @throws Exception on failure
+      std::string downloadFile(const std::string& filePath) const;
+
       /// @brief Use this method to remove webhook integration if you decide to switch back to getUpdates.
       /// Returns True on success.
       /// @param dropPendingUpdates: Pass True to drop all pending updates.

src/Api.cpp

@@ -1,3 +1,6 @@
+#include "cpr/accept_encoding.h"
+#include "cpr/response.h"
+#include "cpr/status_codes.h"
 #include "cpr/timeout.h"
 #include <exception>
 #include <tgbotxx/Api.hpp>
@@ -13,7 +16,7 @@ nl::json Api::sendRequest(const std::string& endpoint, const cpr::Multipart& dat
                          // You can initiate multiple concurrent requests to the Telegram API, which means
                          // You can call sendMessage while getUpdates long polling is still pending, and you can't do that with a single cpr::Session instance.
   bool hasFiles = std::any_of(data.parts.begin(), data.parts.end(), [](const cpr::Part& part) noexcept { return part.is_file; });
-  if(hasFiles) // Files can take longer to upload
+  if (hasFiles)// Files can take longer to upload
     session.SetTimeout(FILES_UPLOAD_TIMEOUT);
   else
     session.SetTimeout(TIMEOUT);
@@ -319,7 +322,7 @@ Ptr<Message> Api::sendAudio(std::int64_t chatId,
     data.parts.emplace_back("performer", performer);
   if (not title.empty())
     data.parts.emplace_back("title", title);
-  if(thumbnail.has_value()) {
+  if (thumbnail.has_value()) {
     if (thumbnail->index() == 0) /* cpr::File */ {
       const cpr::File& file = std::get<cpr::File>(*thumbnail);
       data.parts.emplace_back("thumbnail", cpr::Files{file});
@@ -367,7 +370,7 @@ Ptr<Message> Api::sendDocument(std::int64_t chatId,
   }
   if (messageThreadId)
     data.parts.emplace_back("message_thread_id", messageThreadId);
-  if(thumbnail.has_value()) {
+  if (thumbnail.has_value()) {
     if (thumbnail->index() == 0) /* cpr::File */ {
       const cpr::File& file = std::get<cpr::File>(*thumbnail);
       data.parts.emplace_back("thumbnail", cpr::Files{file});
@@ -401,3 +404,26 @@ Ptr<Message> Api::sendDocument(std::int64_t chatId,
   Ptr<Message> message(new Message(sentMessageObj));
   return message;
 }
+
+Ptr<File> Api::getFile(const std::string& fileId) const {
+  cpr::Multipart data{};
+  data.parts.reserve(1);
+  data.parts.emplace_back("file_id", fileId);
+  nl::json fileObj = sendRequest("getFile", data);
+  Ptr<File> file(new File(fileObj));
+  return file;
+}
+
+std::string Api::downloadFile(const std::string& filePath) const {
+  std::ostringstream oss;
+  oss << BASE_URL << "/file/bot" << m_token << "/" << filePath;
+
+  cpr::Url url{oss.str()};
+  cpr::Timeout timeout{FILES_UPLOAD_TIMEOUT};
+  cpr::AcceptEncoding encoding = cpr::AcceptEncoding{{cpr::AcceptEncodingMethods::deflate, cpr::AcceptEncodingMethods::gzip, cpr::AcceptEncodingMethods::zlib}};
+  cpr::Response res = cpr::Get(url, timeout, encoding);
+  if (res.status_code == cpr::status::HTTP_OK) {
+    return res.text;
+  }
+  throw Exception("Failed to download file " + filePath + " with status code: " + std::to_string(res.status_code));
+}