File input inside a form improvements

[raquelarrojo]

Checklist
(Check off all the items before submitting)

• Build process is done without errors and all tests pass in /lib directory.
• Self-reviewed the code prior to submitting.
• Meets accessibility standards.
• Added/updated documentation to /website as needed.
• Added/updated tests as needed.

Purpose
Improve file input's documentation and behavior.

Description

• Improved callbackFile description in props table.
• Removed name from the input file to avoid that inconsistent data is received in submit inside a form.
• Added type="button" to file item's delete button, since by default button type is submit. This would cause a problem when the file input is inside a form.
• Changes basicUsage example to print files stateful variable instead of input file' files. Input file is uncontrolled and files are read only, so they do not contain files updated.
• Added file input inside a form example.
• Remove file input inside a form test, since we are not controlling what is arriving in submit event.

Closes #1492