I have explained Atlassian the severity of the problem. I hope they share my opinion, and fix the problem in next version of JIRA. The problem is that nothing can't be done by JWT team to solve this problem, since there are no alternatives to access transition attachments but the affected API.
In case I don't get news from Atlassian about fixing the problem in next version of JIRA, I will be forced to declare compatibility with a BIG WARNING, explaining the impacted functionalities, which are conditions, validations and post-functions that has some reference to attachments uploaded in a transition screen.
With this information, you can install JWT 2.2.42 on your JIRA 7.5.0, in case you are not using affected functionalities, or you can live with the problem.
BTW, you can help to solve the problem by voting on issue JRASERVER-65939. Thanks.
JIRA Workflow Toolbox has been declared compatible with JIRA 7.5.0, although affected by JRASERVER-65939. We can't keep JWT blocked in JIRA 7.4 because of a bug on JIRA 7.5 we can't do anything to solve, except report it to Atlassian, and try to influence to get it fixed as soon as possible.
The following features are currently impacted by the bug in JIRA 7.5.0:
Transition's attachments with details.
Number of transition's attachments.
Attachments: attachments uploaded in transition screen are missing.
Attachments with details: attachments uploaded in transition screen are missing.
Number of attachments: attachments uploaded in transition are not counted.
Recent attachments: attachments uploaded in transition screen are missing.
Recent attachments with details: attachments uploaded in transition screen are missing.
Send an email: attachments uploaded in transition screen are missing in emails sent.