Well, whenever you try to upload a file in a format other than acceptable, you will definitely get this error. By default, WordPress allows you to upload images in .gif, .png, .jpg, and .jpeg formats; documents in .xls and .xlsx, .pdf, .ppt, .pptx, .pps and .ppsx, .doc and .docx and .odt formats; audio in .flv, .mp3, .m4v, .wma, .m4a, .mid, .ogg, .mp4, .wma, and .mid audio formats, as well as .ogv, .mp4, .3gp, .m4v, .avi, .mov, .3g2, .webm, .wmv and .mpg. In addition, other file formats are allowed, such as .zip and .key. Thus, whenever you try to upload a file in a format other than the above, WordPress gives an error. Now let's see how to fix it.