原文转载自：https://www.zhihu.com/question/27971703   作者：pezy
根据 Appending Binary Files Using the COPY Command 可粗略的认为，就是以二进制的方式合并两个文件到一个新文件中。

举例说明：

echo Hello > 1.txt
echo World > 2.txt
copy /b 1.txt+2.txt 3.txt

那么此刻 3.txt 中应该是

Hello
World

如果仅仅实现上述例子中的基本功能，使用几个系统的 API 就可以搞定。代码如下：

#include <windows.h>
#include <stdio.h>

void main()
{
	HANDLE hFile;
	HANDLE hAppend;
	DWORD  dwBytesRead, dwBytesWritten, dwPos;
	BYTE   buff[4096];

	// Copy the existing file.

	BOOL bCopy = CopyFile(TEXT("1.txt"), TEXT("3.txt"), TRUE);
	if (!bCopy)
	{
		printf("Could not copy 1.txt to 3.txt.");
		return;
	}

	// Open the existing file.

	hFile = CreateFile(TEXT("2.txt"), // open 2.txt
		GENERIC_READ,             // open for reading
		0,                        // do not share
		NULL,                     // no security
		OPEN_EXISTING,            // existing file only
		FILE_ATTRIBUTE_NORMAL,    // normal file
		NULL);                    // no attr. template

	if (hFile == INVALID_HANDLE_VALUE)
	{
		printf("Could not open 2.txt.");
		return;
	}

	// Open the existing file, or if the file does not exist,
	// create a new file.

	hAppend = CreateFile(TEXT("3.txt"), // open 3.txt
		FILE_APPEND_DATA,         // open for writing
		FILE_SHARE_READ,          // allow multiple readers
		NULL,                     // no security
		OPEN_ALWAYS,              // open or create
		FILE_ATTRIBUTE_NORMAL,    // normal file
		NULL);                    // no attr. template

	if (hAppend == INVALID_HANDLE_VALUE)
	{
		printf("Could not open 3.txt.");
		return;
	}

	// Append the first file to the end of the second file.
	// Lock the second file to prevent another process from
	// accessing it while writing to it. Unlock the
	// file when writing is complete.

	while (ReadFile(hFile, buff, sizeof(buff), &dwBytesRead, NULL)
		&& dwBytesRead > 0)
	{
		dwPos = SetFilePointer(hAppend, 0, NULL, FILE_END);
		LockFile(hAppend, dwPos, 0, dwBytesRead, 0);
		WriteFile(hAppend, buff, dwBytesRead, &dwBytesWritten, NULL);
		UnlockFile(hAppend, dwPos, 0, dwBytesRead, 0);
	}

	// Close both files.

	CloseHandle(hFile);
	CloseHandle(hAppend);
}

将代码中的 1.txt, 2.txt, 3.txt 改为任何别的文件，是一样的。如题主补充的要求，做如下替换即可：

1.txt -> a.jpg
2.txt -> b.txt
3.txt -> c.txt (文本)，c.jpg(更改后缀，成为图片)

诸如 jpg 之类的格式，依然是二进制文件，经过渲染，表现为图片而已。