Allow multiple files to upload for Haystack UI

[MarkusSagen]

When using Haystack UI and streamlit, the default behavior is to upload one file at at time. Trying to upload a new file will override any file one has already uploaded. Streamlit now supports uploading multiple files and may be more intuitive for users of Haystack to use it as the default behavior.

Return type for st.sidebar.file_uploader when accept_multiple_files=True is a list of the files and empty if no files are provided

Proposed changes:

• Change st.sidebar.file_uploader to default to accept_multiple_files=True to allow for multiple files to be uploaded.

Status (please check what you already did):

• First draft (up for discussions & feedback)
• Final code

[tholor]

Great addition! Thanks @MarkusSagen

Streamlit now supports uploading multiple files

What's the minimum version that is needed here so that we can specify it in the requirements?

[MarkusSagen]

Thank you @tholor!
Forgot to add that. I'm not quite sure, since they don't specify releases by the issues they resolve, making it a bit tricky to track.
It seems that at least v.0.68.0 would be needed for Streamlit
But there have been some reported issues with uploading multiple files and were resolved in Version 0.76.0

[tholor]

Ok great. That's then the required version is actually not too recent. I believe we had another feature that required 0.84 so I changed it to >= 0.84.0

Ready to merge :)

[MarkusSagen]

Great! Thanks :)