File Joiner

File Joiner puts a split file back together. Drop in the numbered pieces, such as movie.mp4.part001, movie.mp4.part002, and movie.mp4.part003, and the tool checks that every piece is present, joins them in the right order, and gives you the original file as one download. It is the reverse of the File Splitter on this site, and it reads the same naming scheme: the original file name, then .part, then a number.

The order of the parts matters completely, and the tool never trusts the order you added them in. When you select several files in a file picker or drag them out of a folder, browsers and operating systems do not promise that they arrive in the order you see on screen. Sorting by name as plain text is also a trap, because part10 would land before part2. File Joiner reads the number at the end of each name and sorts by its numeric value, so part2 comes before part10 and part002 before part010, whatever the order of the drop and however many digits the numbers were written with.

Before anything can be downloaded, the tool checks that the numbers are contiguous. If you have part001 and part003 but part002 is missing, it shows a red row between them that names the missing part, explains the problem above the list, and keeps the join button disabled. A file joined with a hole in the middle would be shorter than the original, and for a video, an archive, or a disk image it would be silently corrupt, so the tool refuses instead of guessing. It also catches a set that starts at part002 because part001 is absent, and the same number added twice, such as two copies of part003 from different folders.

Names alone have one blind spot. If you have parts 001, 002, and 003, there is no way to tell from the file names whether the file was split into three parts or four. That is why the tool accepts the .manifest.txt file the File Splitter writes next to its parts. Drop it in with the parts and the tool knows the exact number of parts and the exact size of the original in bytes. A missing last part then shows up as a missing row, and a part that was cut short by an interrupted copy shows up as a size mismatch, with the joined file still blocked until the set is complete.

The joining itself is fast and light. The tool builds the result with the browser's Blob constructor from the part files, which references the files on your disk instead of copying them into memory. Building the joined file is instant at any size, and the only real wait is your browser writing it to disk, so there is no size limit beyond what your disk can store. Nothing is uploaded at any point. The parts are read from your disk only when your browser writes the finished download, and no request carries a file name or any of its bytes.

If you would rather use the command line, the commands in the File Splitter manifest do the same thing: cat over the part files on macOS and Linux, or copy /b in Command Prompt on Windows. File Joiner is for the times when you are on a locked-down or shared machine, when you do not want to type a command, or when you want the missing-part check done for you.