Hi Matthias,
By design possibility to add / remove / edit attachments requires editing rights. By default it's even stricter - only person which added attachment can remove it, and in exactly same step where it has been added.
You could though solve it in 2/3 ways, maybe someone will come up with another ones:
1. Give sales employee edit rights, but create edit restrictions on fields/paths (if fields are in group, then group level restriction is enough), something like 'When user doesn't have a task' - should be possible using 'Assigned persons', and 'Current user is one of'. It is a bit of work though.
2. Create a 2 step subworkflow, which will be used to add attachments to parent - create a global* 'hyperlink start element'** action, and use related attachments *** tab to display them.
3. Is an variation on 2 - creating subworkflow, but after registering that subworkflow, you could use move workflow **** action - trigger it on child, and target parent workflow. Add technical path on each step of parent workflow, with Add Attachment ***** action - this should target latest child workflow for attachment adding.
I'd probably try to work with 2nd approach, as it looks like least pain to maintain it.
Also - you could try a variant with only 1 subworkflow dedicated to keep attachments added 'during process' instead of creating a subworkflow each time attachments are added.
Note from my perspective - usually letting someone add information during the process, when it's not their time to act without proper information flow might lead to errors.
If someone from Sales have a new attachment, maybe a better way would be to give everyone possibility to move workflow to Sales, where they can attach what is needed, and use 'Back to the previous step' destination step on a path in sales.
It forces Sales to inform someone that there is some kind of a new data, because they have to ask current assignees to pass them the instance.
* https://docs.webcon.com/docs/2023R3/Studio/Workflow/Workflow_GlobalAction3/
** https://docs.webcon.com/docs/2023R3/Studio/Action/Form/Action_Hyperlink
*** https://docs.webcon.com/docs/2023R3/Studio/Process/Attribute/Basic/Itemlist/BasicColumns/RelativeAttachmentsConfig
**** https://docs.webcon.com/docs/2023R3/Studio/Action/Workflow/MoveManyWorkFlows/
***** https://docs.webcon.com/docs/2023R3/Studio/Action/Attachments/Action_AddAttachment