@CarinaEA so with Forms itself you're limited as to what type of files you can also request and as you know, it's only available to people within your own 365 tenancy. So, another other way of approaching this situation is to send them a File Request.
Two considerations you need to factor in:
1. You will need to request an email address on the Form in order to send the 'File Request' to.
2. Since Forms itself still doesn't have an in-built email address verification process, it's highly advisable to request the Form submitter to 'confirm' their email address. (You'll use this later in Power Automate.)
As an example of what I've done before with such a Form is to incorporate the following sections and branching rules:
1. Prompt for an initial email address.
2. Ask if they have files they need to send.
3. If yes, using branching within the Form to direct them to the confirm the email address.
Once you've captured these details, you can then have a simple Power Automate process that sends them an email with a URL to your 'File Request'.
A few things to note here within the Power Automate process:
1. Convert the supplied email addresses into lowercase, so when the 'Condition' is checking, the two email address fields are at least in the same case... bob<at>outlook<dot>com is different to Bob<at>outlook<dot>com when using a 'match'.)
2. In the event that the person completing the Form hasn't typed the two email addresses correctly, I'll alert someone internally. Hopefully with the supplied name and company name someone will be able to recognize the person and manually send them an email with the Request File URL.
Later if you wanted, you can take Power Automate a little further, waiting and checking for when files have been uploaded, moving them to a more suitable location and notifying the relevant person, etc., etc...
Anyhow, hope this is of some use.