Use an Upload field when respondents should send files with their answers, such as a CV, a photo or a few supporting documents.

Upload selected in the designer, with its properties open.
Set up the field
Upload is under Input Fields. Under File Types, tick the groups or types you want to accept, such as Documents or Images. If nothing is ticked, any file is accepted. For other extensions, such as txt, use Add custom file types.
Then set the limits. Min. number of uploads and Max. number of uploads decide how many files a respondent can add. Leave the minimum at 0 to keep the field optional. File size limit (MB) applies to each file; on the Free plan it can be at most 100 MB.
What respondents see
On the published form, respondents drag files onto the drop area or pick them with browse. A file that does not fit the limits shown there is not added.

The drop area lists the allowed file types, the number of files and the maximum file size.
The form can only be sent once every file has finished uploading.

The third file is still uploading.
If the form uses Save and resume progress, uploaded files come back with the draft. They are kept for about a day.
Download the files
Download the files from the submissions page, one field at a time. A single file downloads as it is; several files come as one zip. Want a CV and a photo as separate downloads? Give each its own Upload field.

Several files in one field download as a single zip.
Upload space
Uploaded files, and the images you place on your forms, count towards your plan’s upload space. Check the Upload space meter in the dashboard’s Monthly Overview. When it is full, new uploads are refused.