Support for pre-compressed and ETag in download

[JosePineiro]

When downloading file request:

1. Gzipped file serving:
   • Automatically detects and serves pre-compressed .gz files when uncompressed originals are missing
   • Properly sets Content-Encoding: gzip headers
   • Implements If-None-Match header comparison for 304 (Not Modified) responses (RFC 7232)
   • Implements ETag header using CRC-32 from gzip trailer (bytes 4-7 from end)
   • Optimize for speed Changes affect:
     void AsyncWebServerRequest::send(FS &fs, const String &path, const char *contentType, bool download, AwsTemplateProcessor callback) AsyncWebServerResponse *
     AsyncWebServerRequest::beginResponse(FS &fs, const String &path, const char *contentType, bool download, AwsTemplateProcessor callback)
     AsyncFileResponse::AsyncFileResponse(FS &fs, const String &path, const char *contentType, bool download, AwsTemplateProcessor callback)
     : AsyncAbstractResponse(callback)

[JosePineiro]

Implemented as #219

[Copilot]

Pull Request Overview

This PR enhances file downloads by automatically serving pre-compressed .gz files when originals are missing, adding gzip headers, and implementing CRC32-based ETag handling for 304 responses.

• Fallback to gzip-compressed file in AsyncFileResponse, setting Content-Encoding, ETag, and Cache-Control.
• Refactor AsyncWebServerRequest::send() to check uncompressed first, handle compressed files, and respond 304 Not Modified when ETags match.
• Simplify Content-Disposition logic for inline vs. download modes.

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
src/WebResponses.cpp	Updated AsyncFileResponse constructor to support gzip fallback, ETag generation, and download/inline headers.
src/AsyncWebServerRequest.cpp	Refactored send() to pre-check uncompressed files, open and validate .gz files, and implement ETag-based 304 logic.

Comments suppressed due to low confidence (1)

src/WebResponses.cpp:716

• No tests cover the scenario where a corrupted or invalid gzip file triggers a 404 response; add unit tests to validate this path.

      _code = 404;

[Copilot]

The download flag is ignored when serving compressed files. You should skip serving the .gz version if download is true to respect the download mode.

Suggested change

	const String gzPath = path + asyncsrv::T__gz;
	const String gzPath = path + asyncsrv::T__gz;
	// Skip serving compressed file if download is true
	if (download) {
	send(404);
	return;
	}

[mathieucarbou]

Thanks @JosePineiro ! Let's get this PR merged, then I will cut a new release.

[JosePineiro]

Thanks @JosePineiro ! Let's get this PR merged, then I will cut a new release.

Tomorrow I'll submit another PR with a speed improvement for the _setContentTypeFromPath function.
The improvement is between 2 and 5 times.
You may want to wait before releasing a new release.