If we disable the AttachmentManager cache how will it affect the system, how will the data stream be handled?

The attachment manager cache is mainly used when you open from library or saving large amounts of data. Therefore -  disabling cache will result in extra work for the library database.

Please note - disabling attachment manager cache will not stop temporary files from being created in that location. Disabling cache only stops caching data fetched from Spotfire library and information link results. Disabling attachment manager cache will only reduce size of AttachmentManager folder size by size of the cache (which is around 10 GB by default). 

However, for normal functionality of Spotfire Attachment Manager itself still needs to work, attachments are created as soon as "data" is downloaded from or uploaded to the Spotfire Server. The creation of these attachments is essential for the operation of the Spotfire Server and cannot be disabled.

In the linked documentation page 
https://docs.tibco.com/pub/spotfire_server/10.3.0/doc/html/TIB_sfire_server_tsas_admin_help/GUID-0639974D-AF4B-4267-83D6-BF0CAF36237C.html
there are separate settings for library and information services. If you set information services cache to false that means that you have globally disabled caching for information services. But without changing that setting you still control each information link cache setting and lifetime when you set them up.

How does the data flow of information links work - will it first be fully stored as a temp file by the AttachmentManager and then passed to Analyst or Web Player?

The data flow for information links work will not be affected by disabling AttachmentManager cache, data will still go through the server and then be passed to Analyst or Web Player, not as single large temp file but in small parts which are later removed if you have not enabled caching on that information link. Setting information services caching to false will mean that the results of information link executions are not being stored (on global level even if you have caching enabled on individual information link level). 

When disabling the AttachmentManager cache will the data go to the JVM? If yes was are the risks of getting more GC?

Disabling attachment manager cache should have no influence on JVM and GC for activity related to information links.
 

What is a normal size for the AttachementManager folder?

The files that you see related to attachment manager depend mostly how you use Spotfire so different number can be normal for different customers. If Spotfire Server is configured to cache Information Links and library content, this uses additional disk space. By default, caching is enabled and the max cache size set to 10 GB. And expiration time is one day by default. You can set cache settings using config-attachment-manager The max-cache-expiration-time value is the maximum idle time (in seconds) after which cache entries are evicted. Setting this parameter to a negative value disables the cache. 

You can set cache settings using config-attachment-manager command
https://docs.tibco.com/pub/spotfire_server/10.3.0/doc/html/TIB_sfire_server_tsas_admin_help/GUID-1361A13B-8FBF-4E28-BB99-680C7F90543C.html

The max-cache-expiration-time value is the maximum idle time (in seconds) after which cache entries are evicted. Setting this parameter to a negative value disables the cache. 

The max-cache-size value is the maximum amount of disk space (in megabytes) used by the cache. Setting this parameter to a negative value disables the cache.

Attachments are created as soon as "data" is downloaded from or uploaded to the Spotfire Server. The data may either be library items or the results of Information Link executions (and a perhaps a few other much less frequent things). The creation of these attachments is essential for the operation of the Spotfire Server and cannot be disabled.

To reduce database load and latency that comes from fetching data from databases (the Spotfire Server database or Information Services data sources) we also have a mechanism through which attachments may be cached (as opposed to being cleaned up as soon as they've been read). This size of this cache can be configured (and it can even be completely disabled) but that won't stop the creation of attachments.

By default all library items that are downloaded or uploaded are cached. The results of Information Link executions can also be cached (and then uses the Attachment Cache underneath) but that needs to be enabled per Information Link (since we for Information Links, unlike library items, do not have a generic way of detecting if the cached data is stale or not).
 

We see older files then today in the AttachmentManager folder, when will it clean up?

By default items will be evicted from the cache if they've been idle for more than 24 hours, or if a new item is added and there is no more room in the cache (based on the configured maximum size - which by default is 10240 MB). In the latter case the least recently used items are evicted.

As for old items being present that is fully expected if those items are frequently requested (and thus never becomes "idle").
 

What happens if the disk is full and the AttachmentManager cannot write?

As for disk space, in general not enough disk space will be problematic for the functionality of Spotfire server. What you can do is to set the value for The max-cache-size which will be the maximum amount of disk space (in megabytes) used by the cache.