"Send" in new web client triggers file upload block

We are happy to answer your questions about the code or discuss technical ideas.

Please complete the following checklist (by adding [x]):

• I have searched open and closed issues for duplicates
• This isn't a feature request
• This is not a report about my app not working as expected

When I attempt to send an email using the new web client, it triggers a file upload block in my company's security software. This affects all web client versions since version 4.x, including the current version 5.0.1.4 beta. This does not affect web client version 3.x.
This used to be served by old.protonmail.com, but not any more (as of today).

I'll put a little more information about the behavior and mitigations below.

I'd like to ask the community for information on two questions;

1. What are the differences in send behavior that were introduced in web client 4.x? Looking for enough info to suggest ruleset changes to my infosec team so as not to trigger this file upload block behavior.
2. Is there a way for me to pull sources from the latest 3.x client and create a local instance of the client here? How can I do that?

Thank you for any information.

/cb

To address the problem, I have:

1. Used old.protonmail.com to send emails
2. Begged Support for specific information that may allow me to address this with my company infosec people (all info requests refused, though they did point me to this github page)
3. Begged Support to keep old.protonmail.com active while I try to work this (refused, as of today)
4. Sent an blast to various known email addresses at ProtonMail (contact@protonmail.com, etc.) begging them to keep old.protonmail.com active. They passed the buck to Support.

Some possibly relevant facts about the behavior:

1. Same behavior on Edge and Firefox, the only browsers available in my environment.
2. Same behavior after clearing cache, rebooting, etc. etc.
3. In the new client I can:
   • Read emails
   • Search emails
   • Compose messages
   • Save messages to Drafts
4. In the new client I cannot:
   • Embed an image or add an attachment. This is not a problem -- policy prohibits file uploads and I can live without this functionality. Attempting this triggers the same web file upload block message from the security software.
   • Send an email. In the old client, version 3.x this works fine. In any version since 4.x, including the current version 5.0.1.4 beta, it triggers the file upload block message.

Thanks again for any help with this.

/cb

The report names no source files, tests, or entry points. Start by reproducing the send and attachment behavior in the 3.x and 4.x web clients using the listed Edge or Firefox environment, then compare the relevant client behavior. Done would require identifying the changed send behavior and documenting enough information for the infosec ruleset request.