Forms are often used to register, login, and upload files.
Lots of sanity checks can be done in JavaScript in the browser before the upload or on the server after the upload.
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 command

curl -X POST -F 'fileToUpload=@./picture.jpg'