Node JS delete multiple MySQL linked records and delete physical file
I am not primarily a JS developer, therefore I won't assist you with the JavaScript part itself, but I can give you a few hints regarding several issues I've found.
Lack of transactional safety
In your example, you're executing two DELETE statements on the database.
1) delete t1, t2 from postschema t1 join commentschema t2 on t1.id = t2.postid where t1.id = ?

2) delete from postschema where id = ?

Those statements are isolated in their own transactions and commit right after their execution. The problem here is, if the statement 1) succeeds, but 2) fails, all the records deleted with the statement 1) will remain deleted, which can easily lead to unexpected database state.
In order to prevent this, you want to wrap the statements which should either all succeed or all fail in a database transaction.
DELETE statement ambiguity and difficult readability
Although MySQL supports deleting records through JOIN, effectively allowing you to delete data from multiple tables in a single go, such statements are difficult to track what gets actually deleted. On top of that, certain database engines do not support such mechanism at all.
I highly recommend you to change the statement, so that it deletes only the comments, which makes the query much easier to follow and to read.
DELETE FROM commentschema WHERE postid = ?

Removing a file from the FS before removing a database record
This may introduce an unintentional side effect, if deletion of database records fails, leading to a situation where the database record points to a file location which no longer exists.
In order to solve this, the order of operation should be swapped:

delete data from the database,
(now that nothing points to the file) delete the file from the file system.

I have also noticed you're loading images for comments (plural!) by postId, but only removing the image of the first comment in the returned collection.
This may be an accidental error in your implementation, since later you're deleting all comments, which makes me believe you want to delete all images, too.
Code nesting
Your code is quite nested, due to callbacks. In order to resolve this, you can:

introduce smaller functions with different responsibilities (e.g. a function to load data from the database, a function to delete a file,...),
convert functions which output result through callbacks to promises and cleanup the code a little bit through promise-chaining (even better if you use async/await).