Here is a list of all files with brief descriptions:

src
fuzz
fuzz_auth_header.c	Direct fuzzer for the "Authorization:" header parsers
fuzz_common.h	Shared, header-only fuzzing driver for the MHD in-process fuzzers
fuzz_eventloop.c	In-process fuzzer for MHD's external event loop and for the scheduling of the connection life cycle
fuzz_memorypool.c	Direct in-process fuzzer for src/microhttpd/memorypool.c
fuzz_options.c	In-process fuzzer for MHD's daemon option handling and for the daemon start-up / shutdown paths
fuzz_postprocessor.c	Fuzzer for MHD_post_process()
fuzz_request.c	End-to-end in-process fuzzer for the MHD request parser
fuzz_str.c	Direct in-process fuzzer for the string primitives of mhd_str.c
fuzz_tls.c	In-process fuzzer for MHD's HTTPS/TLS integration layer
include
autoinit_funcs.h
mhd_options.h	Additional automatic macros for MHD_config.h
microhttpd.h	Public interface to libmicrohttpd
platform.h	Platform-specific includes for libmicrohttpd
microhttpd
basicauth.c	Implements HTTP basic authentication methods
basicauth.h
connection.c	Methods for managing connections
connection.h	Methods for managing connections
connection_https.c	Methods for managing SSL/TLS connections. This file is only compiled if ENABLE_HTTPS is set
connection_https.h	Methods for managing connections
daemon.c	A minimal-HTTP server library
digestauth.c	Implements HTTP digest authentication
digestauth.h
gen_auth.c	HTTP authorisation general functions
gen_auth.h	Declarations for HTTP authorisation general functions
internal.c	Internal shared structures
internal.h	MHD internal shared structures
md5.c	Calculation of MD5 digest as defined in RFC 1321
md5.h	Calculation of MD5 digest
md5_ext.c	Wrapper for MD5 calculation performed by TLS library
md5_ext.h	Wrapper declarations for MD5 calculation performed by TLS library
memorypool.c	Memory pool
memorypool.h	Memory pool; mostly used for efficient (de)allocation for each connection and bounding memory use for each request
mhd_align.h	Types alignment macros
mhd_assert.h	Macros for mhd_assert()
mhd_bithelpers.h	Macros for bits manipulations
mhd_byteorder.h	Macro definitions for host byte order
mhd_check.h	Macros for MHD_CHECK_(), always-compiled memory-safety invariants
mhd_compat.c	Implementation of platform missing functions
mhd_compat.h	Header for platform missing functions
mhd_itc.c	Implementation of inter-thread communication functions
mhd_itc.h	Header for platform-independent inter-thread communication
mhd_itc_types.h	Types for platform-independent inter-thread communication
mhd_limits.h	Limits values definitions
mhd_locks.h	Header for platform-independent locks abstraction
mhd_md5_wrap.h	Simple wrapper for selection of built-in/external MD5 implementation
mhd_mono_clock.c
mhd_mono_clock.h	Internal monotonic clock functions implementations
mhd_opt_matrix.c	The environment-driven daemon option matrix shared by the tests
mhd_opt_matrix.h	The environment-driven daemon option matrix shared by the tests
mhd_panic.c
mhd_panic.h	MHD_panic() function and helpers
mhd_panic_tripwire.h	Test-only tripwire that turns a fatal libmicrohttpd invariant failure into a loud, greppable, machine-classifiable test error
mhd_send.c	Implementation of send() wrappers and helper functions
mhd_send.h	Declarations of send() wrappers
mhd_sha256_wrap.h	Simple wrapper for selection of built-in/external SHA-256 implementation
mhd_sockets.c	Implementation for sockets functions
mhd_sockets.h
mhd_str.c	Functions implementations for string manipulating
mhd_str.h	Header for string manipulating helpers
mhd_str_types.h	Header for string manipulating helpers types
mhd_threads.c	Implementation for thread functions
mhd_threads.h	Header for platform-independent threads abstraction
postprocessor.c	Methods for parsing POST data
postprocessor.h	Declarations for parsing POST data
reason_phrase.c	Tables of the string response phrases
response.c	Methods for managing response objects
response.h	Methods for managing response objects
sha1.c	Calculation of SHA-1 digest as defined in FIPS PUB 180-4 (2015)
sha1.h	Calculation of SHA-1 digest
sha256.c	Calculation of SHA-256 digest as defined in FIPS PUB 180-4 (2015)
sha256.h	Calculation of SHA-256 digest
sha256_ext.c	Wrapper for SHA-256 calculation performed by TLS library
sha256_ext.h	Wrapper declarations for SHA-256 calculation performed by TLS library
sha512_256.c	Calculation of SHA-512/256 digest as defined in FIPS PUB 180-4 (2015)
sha512_256.h	Calculation of SHA-512/256 digest
sysfdsetsize.c	Helper for obtaining FD_SETSIZE system default value
sysfdsetsize.h	Helper for obtaining FD_SETSIZE system default value
tsearch.c
tsearch.h
tools
mhd_tool_get_cpu_count.c	Implementation of functions to detect the number of available CPU cores
mhd_tool_get_cpu_count.h	Declaration of functions to detect the number of available CPU cores
mhd_tool_str_to_uint.h	Function to decode the value of decimal string number