Add the ability to set a 'response-content-disposition' param/header when uploading a file

[dylanwolff]

Per a client request (see: https://3.basecamp.com/3810878/buckets/9505978/todos/1362900706) it would be useful to be able to set a response-content-disposition param when uploading a file via an upload field, as a means of controlling whether the file is rendered inline or as an attachment when a user clicks a link to access it (by setting the Content-Disposition header appropriately for the file on S3).

See also some internal discussion here: https://icelab.slack.com/archives/C2BT34RK7/p1541993717013500

[makenosound]

So the use case here is that:

• For a certain type of file
• In a certain upload context

We want to change the content-disposition we set? There are a few different ways we could handle, so just want to check that both those statements above are true or not. Think we could potentially make it:

• A global configuration for formalist (which we can adjust per instance of an entire form)
• A field-level configuration so that if you had two upload fields in a single form you could set different content disposition rules for each.

[dylanwolff]

I mentioned to the client that we could do this either at a global level for particular types of files (which I thought might be the simpler option) or potentially also on a per-upload-context basis.

The client's response was:

Thanks for that information - I think if we could define it just for PDF's (which will be the most used file) that would be great

So if the global configuration option is going to be much simpler to implement perhaps that will suffice for now? We could potentially then add the ability to set this at the field level at a later date.

[makenosound]

Yeah, we could perhaps make it a tiered thing.

1. Some default configuration (per file type)
2. Ability to adjust that global configuration when instance is created
3. Ability to override the global configuration per upload_field instance