That should be returned when you're attempting to upload an invalid file type via the API.
What filetype were you attempting to upload? Does your filetype and contents match?
DocuSign supports the following file types:
DOCUMENT .as, .asl, .asp, .doc, .docm, .docx, .dot, .dotm, .dotx, .htm, .html, .pdf, .pdx, .rtf, .txt, .wpd, .wps, .wpt
DRAWING .dwg, .dxf, .emz, .svg, .svgz, .vdx, .vsd, .vss, .vst IMAGE .bmp, .cdr, .dcx, .gif, .ico, .jpg, .jpeg, .pct, .pic, .png,
.rgb, .sam, .tga, .tif, .tiff, .wpg
PRESENTATION .dps, .dpt, .pot, .potx, .pps, .ppt, .pptm, .pptx
SPREADSHEET .csv, .et, .ett, .xls, .xlsx, .xlt
Note: DocuSign does not support macro-enabled files. To send a macro-enabled file through DocuSign, you must first convert it to PDF
format.
File size limitations
Along with the file format, DocuSign has file size limitations for
your file uploads:
MAXIMUM FILE SIZE 25 MB
RECOMMENDED SIZE 5 MB
For best performance, DocuSign recommends restricting your file
uploads to no more than 5 MB.