Image upload not working. Gives an error: Undefined while uploading the image

When a user tries to upload an image from the front end form, it gives an error: undefined after the image progress reaches 100%. Please note that I have set the max size for the attachment as 102400 kb (100 Mb) which means practically any image can be easily uploaded even if it is in Ultra HD quality. Still can not get it to work. The images under 1 Mb are easily being uploaded but not beyond that. Also, the uploaded image is not being set as featured image even after using the checkbox in settings dashboard. Link to the form on my website is: http://easymobileshop.com/post_ad. Please check and fix.