File format spec for TreeSheets .cts files, version 23. This should be enough for a programmer to write an importer/exporter.

struct File     // not really a struct, i.e. no alignment, read an element at a time
{
    char magic[4] = "TSFF";
    char version = 23;
    // TreeSheets saves the selection and the zoom level when the file is saved.
    int xs; // xs is the number of cells within the selection in x direction
    int ys; // ys is the number of cells within the selection in y direction
    int zoomlevel; // zoom level of the document
    
    Image images[0 or more];

    char ident = 'D';   // marks start of document, and end of list of images

    // the remainder of the file from here is written as a zlib compressed stream
    Cell root;

    for (;;) {
        String tagname; // terminated by empty string];
        int tagcolor;
    }
}

struct Image
{
    char ident;   // marks start of image
    // Character value depends on image type, i.e. 'I' for PNG and 'J' for JPEG
    double display_scale; // display scale of image
    int64_t imagelen; // length of image (8 bytes)
    char image_data[len]; // whatever format wxImage::LoadFile uses
}

struct Cell
{
    uchar celltype;     // enum { CT_DATA = 0, CT_CODE, CT_VARD, CT_VIEWH, CT_VARU, CT_VIEWV };
    uint cellcolor;
    uint textcolor;
    uchar drawstyle;    // enum { DS_GRID, DS_BLOBSHIER, DS_BLOBLINE };
    uchar cellcontents; // enum { TS_TEXT = 0, TS_GRID, TS_BOTH, TS_NEITHER };
    // The first cell that is within the selection is marked with the highest bit set to 1 (1 x x x x x x x) in cellcontents.
    if (TS_TEXT or TS_BOTH)
    {
        String text;
        uint relativesize;
        uint imageindex; // or 0xFFFFFFFF for no image
        uint stylebits;   // enum { STYLE_BOLD = 1, STYLE_ITALIC = 2, STYLE_FIXED = 4, STYLE_UNDERLINE = 8, STYLE_STRIKETHRU = 16 };
        uint64 lastedit;    // internal representation of wxDateTime
    }
    if (TS_GRID or TS_BOTH)
    {
        uint xs, ys;
        uint bordercolor;
        uint user_grid_outer_spacing;
        uchar verticaltextandgrid;
        uchar folded;
        uint columnwidths[xs];
        Cell cells[xs * ys]; // row major
    }
}

struct String  // as wxDataOutputStream::WriteString writes it, apparently UTF-8 prefixed by length
