Fix bug. Sometimes the content type isn't sent.

[JosePineiro]

Fixes an regresive bug where Content-Type was not properly set when serving files, which specifically affected Firefox when using "view page source" functionality. The issue primarily manifested in Firefox's view-source feature, but the fix ensures consistent behavior across all browsers and use cases.

Changes:
Fixed missing contentType headers
Optimized string handling by replacing PSTR() with static constant

• Optimized string handling by replacing snprintf_P with snprintf
• Changed from dynamic buffer formatting to direct header addition using static template
• Improved memory efficiency and code readability

[Copilot]

Pull Request Overview

Fixes a regression where the Content-Type header could be omitted when serving files (notably in Firefox’s view-source) and optimizes string handling by replacing PSTR calls with static constants.

• Added T_inline and T_attachment literals and removed PSTR/snprintf_P usages
• Inverted the contentType check to correctly set a default when none is provided
• Updated fs.open calls to use explicit fs::FileOpenMode enum for clarity

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/literals.h	Introduced T_inline and T_attachment constants for Content-Disposition headers
src/WebResponses.cpp	Fixed empty contentType logic, swapped snprintf_P for snprintf and static templates
src/AsyncWebServerRequest.cpp	Replaced mode string "r" with fs::FileOpenMode::read in fs.open

Comments suppressed due to low confidence (3)

src/WebResponses.cpp:737

• There are no tests covering the case when contentType is empty. Consider adding a unit test to verify that the default Content-Type is set correctly.

  if (*contentType == '\0') {

src/WebResponses.cpp:748

• [nitpick] The variable name buf is generic. A more descriptive name like dispositionHeaderBuf would improve readability.

    snprintf(buf, sizeof(buf), T_attachment, filename);

src/AsyncWebServerRequest.cpp:30

• Using fs::FileOpenMode::read may affect compatibility with FS implementations expecting a mode string. Ensure all supported file systems accept this enum or update the documentation accordingly.

  File gzFile = fs.open(gzPath, fs::FileOpenMode::read);