CaptainForm is a free online form builder plugin with file upload functionality. Users can collect files from their respondents through form submissions by adding file upload fields to the form.
All supported file extensions of the File Upload field are located in the Advanced section, within the field’s options. To allow only certain file extensions upon upload, at Accept only the following extensions use the drop down list to check the extensions you want to use. 
If you need a new file extension, simply click on Add more extensions.
Here is a list of the file extensions that the File Upload supports: .3gp, .ai, .avi, .bmp, .cat, .cdr, .cod, .cpt, .csv, .dcm, .doc, .docx, .dpt, .dwg, .dxf, .eps, .exp, .flv, .gif, .hwp, .ico, .iges, .igs, .ipt, .jpeg, .jpg, .kit, .kmz, .m4a, .mdl, .mobi, .mod, .mov, .mp3, .mp4, .mpeg4, .msg, .oad, .oadx, .odt, .ogg, .org, .ott, .pdf, .png, .pps, .ppsx, .ppt, .pptx, .prt, .psd, .pub, .rar, .rtf, .slddrw, .sldprt, .step, .stf, .stl, .stl, .stp, .stp, .tif, .tiff, .txt, .wav, .wmv, .wpd, .wps, .x_t, .xls, .xlsx, .zip