Environment: Thunderbird Version: 140.6.0esr (32-bit) OS: Windows 10 Pro 64bit Summary of the Issue: I have encountered a severe issue where Thunderbird sent the wrong attachment content without any warning, despite the user interface showing the correct file. It appears Thunderbird silently substituted the current attachment with an old cached version of a file with the same name sent to the same recipient 8 days prior. Steps that caused the issue: I opened a saved .eml file located on my hard drive by double-clicking it (the file contained an embedded attachment). From the opened message window, I clicked "Forward". I entered the recipient address (Recipient X) and sent the email. Observation before sending: In the composition window, the attachment bar displayed the correct filename. Actual Result (The Error): The email received by Recipient X contained an attachment with the correct filename but the wrong content. Specifically, it attached the content of an old file (sent to the same Recipient X 8 days ago) that happened to have the identical filename. File A (New, in .eml): Filename file.enc, Size 2MB (Correct) File B (Old, sent 8 days ago): Filename file.enc, Size 500KB (Incorrect) Sent Email contained: File B's content (500KB) masked under File A's name. Expected Result: Thunderbird should have extracted and sent the specific attachment embedded in the .eml file I was forwarding, ensuring a unique temporary path to avoid collisions with previous file handles or cached data.