Forms are often used to register, login, and upload files.
Filename and file extension, are often checked, sometime even the file header is chekced to verify wether it's an image or a php file.
A full chapter is dedicated to file upload and filter bypass.
A File field can be identified by the following HTML code
<input type=file name=fileToUpload>:
curl -X POST -F 'fileToUpload=@./picture.jpg' http://10.10.1.11/upload